Backport premailer and diff changes from EE

Backport part of https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/151

See merge request !3318

Expand the issuable sidebar when coming back from the Changes tab

Unless the user explicitly requested the collapsed sidebar.

Fixes: #14353

See merge request !3256

#to_branch_name now uses the iid as postfix

Given the branch name 'mep-mep' with an iid being 1,
the current way, master's way, would yield a branch name of
1-mep-mep. The problem for larger projects however would be that
a developer might forget what iid the issue was.

When this developer would try to tab complete it would:
- Or result in 20+ branches possibly
- Or start with the wrong digit, try again with digit++
  - Would see 20 branches, repeat

Thus the obvious way of solving this is letting the dev tab complete
on the issue title, which is easier to remember.

@DouweM Should this be labelled `pick-in-master`? 

See merge request !3308

Dedupe labels in labels selector in Dashboard pages

Fixes #14418.

See merge request !3311

Fix spacing below edit note form

Also changed to allow the CSS to decide what should be hidden when editing

Thanks @JobV for having eagle eyes 😜 

See merge request !3306

Fixed issue with filtered data persisting when dropdown closed

![labels](/uploads/4b9c902add76263d8488cc34ef49f142/labels.gif)

Closes #14401

See merge request !3302

Add label description in tooltip to labels in issue index and sidebar

The tooltip can get quite big, but it’s so useful to inform people about what these labels mean
new

![Screen_Shot_2016-03-08_at_11.48.28](/uploads/894baaaa1f789c3a2069a924d541e64b/Screen_Shot_2016-03-08_at_11.48.28.png)

![Screen_Shot_2016-03-08_at_11.48.20](/uploads/efe4fa0960054c4071b8f6bf83024403/Screen_Shot_2016-03-08_at_11.48.20.png)

@JobV writes on Slack:
> yeah I think it’s better than not, but I’d have to try it 😃 
> 
> cautious yes, revert if it’s annoying 😉

See merge request !3124

Add information about `image` and `services` field at `job` level in the `.gitlab-ci.yml` documentation

Fixes #14366

/cc @tmaczukin @ayufan @axil

See merge request !3277

Trigger a todo for mentions on commits page

Closes #14006 

* Screenshot:

![todo-commit](/uploads/5d34de0b7afcea7548123dafddf60c45/todo-commit.png)

See merge request !3262

Adds small AJAX optimistic functionality to todos.

Fixes #13656 <br/>
A good first step and boring solution. <br/>
Will make ajax call to remove each issue.  <br/>
If issue is last in group of issues will refresh page.  <br/>
If issues remain in group will remove row with JS.  <br/>
Adds loading spinner to button and disables. <br/>

![todos-optimistic](/uploads/c6aec4a688e5125d4df55a2a3a55d4d7/todos-optimistic.gif) <br/>

cc @dzaporozhets @JobV @dbalexandre 

See merge request !2946

Fix an issue when the target branch of a MR had been deleted

Before displaying the "diverged commits" note, we're checking if the MR
is open, but we should check if it's mergeable instead because this
check ensure the source and target branches exist.

This was introduced by !2217 and fixes #14388.

See merge request !3294

Fix MergeRequest#source_sha when there is no diff

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/14170

### Overview

This MR fixes an unhandled Exception when visiting the page of an open Merge Request  without diff.

### Description

`MergeRequest#source_sha` is expected to return the sha of the source branch last commit. But when an open Merge Request has no diff (e.g. all commits have already been merged to the target branch), `merge_request.source_sha` incorrectly returns `nil`.

This was without consequences before – but since !2217 was merged (a few days ago), it makes `Gitlab::Git::Commit.between` raise an "Unexpected nil argument" exception. This can be reproduced when visiting the http://localhost:3000/gitlab-org/gitlab-test/merge_requests/2 page on a fresh local Gitlab setup.

This MR fixes the crash, by making sure that `source_sha` returns a
correct result even when there is no diff available. I also added tests.

@DouweM I believe you wrote most of this code in the first place ; does this looks correct to you, or is there a better way to resolve this issue maybe?

See merge request !3135

Removed diverging commit count calculation from Repository#build_cache

Using a repository with 1000 branches the old `Repository#build_cache` method would take around 180 seconds to complete. Without calculating the diverging commit counts this method "only" takes around 60 seconds. 

See commit 28cc2413eb5ddf920ce0b5eed803121f8b884754 for more details. This fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/14058

cc @rspeicher 

See merge request !3274

Add avatar to issue and MR pages header

closes #14395
# For Issues

### Before
![Screen_Shot_2016-03-18_at_4.27.25_PM](/uploads/b097c8e304c9737e2f46ee25667829bf/Screen_Shot_2016-03-18_at_4.27.25_PM.png)

### After
![Screen_Shot_2016-03-18_at_4.22.10_PM](/uploads/f7d2b12c9455950c2e660b33e27c1827/Screen_Shot_2016-03-18_at_4.22.10_PM.png)

# For Merge Request

### Before
![Screen_Shot_2016-03-18_at_4.27.09_PM](/uploads/50c7b6ee346f749f448647004ae6031b/Screen_Shot_2016-03-18_at_4.27.09_PM.png)

### After
![Screen_Shot_2016-03-18_at_4.24.17_PM](/uploads/abd37ccf030e0b6d10521a6937206de6/Screen_Shot_2016-03-18_at_4.24.17_PM.png)

See merge request !3301

Fixes issue with any milestone value in dropdown not being selectable

Closes #14293

See merge request !3234

Fixes issue with filter label missing on labels & milestones

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/14407

See merge request !3297

Changing filter dropdowns shows loading

Instead of doing a full refresh of the page - i've modified the filterResults method on the Issues object to work for this form as well

![issues](/uploads/3335f09f7df88b6d419de7fd3d6857d2/issues.gif)

Closes #14359

See merge request !3237

Add confidential issues

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/3678

More information: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/227

See merge request !3282

Prevent a 500 when a repository's root_ref is nil

Closes #14259

See merge request !3278

Improved issue sidebar

Updated the spacing throughout the sidebar so that it fits better on smaller screens. Also adds a more participants button to show any other participants after the first row.

Closes #13353

See merge request !3097

Dashboard labels

Previously because there were no JSON endpoint for labels or milestones
it was fetching HTML and parsing that. This is wrong.

It now fetches from a JSON endpoint.

This also fixes the dashboard/issues page not loading labels or
milestones as the path used to be only for a single project. So instead
I have created a endpoint for labels & milestones JSON on dashboard.

See merge request !3253

Fixes issue with close button not working on MR

Closes #14383

See merge request !3295

Add note in upgrade guide to choose the correct version tag

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/3448

@rspeicher I'm assigning to you since as a release manager you should know better. Let's keep it in this guide for now and then start copying it in the future ones.

See merge request !2284

Show a notice for diffs that are empty/too large

This builds upon https://gitlab.com/gitlab-org/gitlab_git/merge_requests/72 but will fail until said MR has been merged.

Fixes gitlab-org/gitlab-ce#13912, gitlab-org/gitlab-ce#13644, gitlab-org/gitlab-ce#13557, gitlab-org/gitlab-ce#13267, gitlab-org/gitlab-ce#3906

See merge request !3185

[ci skip]

Add side padding to content-list if inside panel
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Fixes #14367, #14349

See merge request !3273

Use relative links in `doc`, fix broken generated HTML links

Links like (doc/web_hooks/web_hooks.md) work in the GitLab source code
web interface, but the HTML generator produces broken links in the `doc`
subdirectories:  
http://doc.gitlab.com/ce/hooks/doc/web_hooks/web_hooks.html  
instead of the right one  
http://doc.gitlab.com/ce/web_hooks/web_hooks.html  
in  
http://doc.gitlab.com/ce/hooks/custom_hooks.html.

Fixes #14338 


See merge request !3261

Remove CI duplication.



See merge request !3290

Improve award emoji test reliability by checking that the emoji is displayed before the search field is focused

See merge request !3288

Check push permissions only when pushing directly to target branch.

Fixes #14374 

See merge request !3280

Fix bug causing dropdown to get cut off in project page

closes #14336 

### Before
![Screen_Shot_2016-03-16_at_11.30.40_PM](/uploads/9f8dd10f0627cdfbd49d7127b233b105/Screen_Shot_2016-03-16_at_11.30.40_PM.png)

![Screen_Shot_2016-03-16_at_11.30.53_PM](/uploads/6dfe64d68867d158a11be7e74cbd1520/Screen_Shot_2016-03-16_at_11.30.53_PM.png)


### After
![Screen_Shot_2016-03-16_at_11.30.14_PM](/uploads/2b87cc17522e8a425a97a78b36982873/Screen_Shot_2016-03-16_at_11.30.14_PM.png)

See merge request !3257

Fix inconsistency in the header of issue and merge request pages

closes #13550 

# For Issue Page

### Before
![Screen_Shot_2016-03-17_at_8.46.59_PM](/uploads/ede7f2b98395362ae87e6258cc10520a/Screen_Shot_2016-03-17_at_8.46.59_PM.png)

### After
![Screen_Shot_2016-03-17_at_8.48.56_PM](/uploads/da7adf17cb397ddc0677a6a5e8033b8e/Screen_Shot_2016-03-17_at_8.48.56_PM.png)

# For Merge request Page

### Before
![Screen_Shot_2016-03-17_at_8.45.45_PM](/uploads/10137827c05c8166a0aa429687e50d1b/Screen_Shot_2016-03-17_at_8.45.45_PM.png)

### After
![Screen_Shot_2016-03-17_at_8.45.01_PM](/uploads/92151c03cfe29b71090a1e0ad31abd6e/Screen_Shot_2016-03-17_at_8.45.01_PM.png)


See merge request !3279

Fixed alignment of download dropdown

With the new dropdown CSS - some dropdowns might look a bit odd

## Previously

![Screen_Shot_2016-03-16_at_11.46.04](/uploads/c1a8fc95ed4410de35d7bd5caedf28f5/Screen_Shot_2016-03-16_at_11.46.04.png)

## Now

![Screen_Shot_2016-03-16_at_11.45.26](/uploads/998637fe0872b2e826a8c17d183499dc/Screen_Shot_2016-03-16_at_11.45.26.png)

See merge request !3245

Fixed filter spacing

Closes #14350

![Screen_Shot_2016-03-17_at_10.21.33](/uploads/5b8f39134cc90ce1dd6565f665a39838/Screen_Shot_2016-03-17_at_10.21.33.png)

See merge request !3267

Bump Git version requirement to 2.7.4

[ci skip]

See merge request !3281

Edited timeago text on comments

Unified the 'edited text' to be the same in descriptions and comments

Closes #5538 

See merge request !3063

Revert color for links inside gray areas

**Before**

![Screen_Shot_2016-03-17_at_12.23.17_PM](/uploads/55e518721d83c3bfc42edba3e97ec44d/Screen_Shot_2016-03-17_at_12.23.17_PM.png)

**After**

![Screen_Shot_2016-03-17_at_12.23.03_PM](/uploads/08fbf8e9139e4eadf191102cffd29617/Screen_Shot_2016-03-17_at_12.23.03_PM.png)

Reverts !3147

See merge request !3276