Fix wiki linking behavior for markdown wiki pages

Related to #18019 

- As per the documentation in !4372 

## TODO
- [ ] !4432 Have wiki linking behave as per the documentation
    - [x] Move `WikiLinkFilter` specs to the pipeline level
    - [x] Verify current behavior on wiki `show` page
    - [x] Fix current behavior on wiki `show` page
    - [x] Verify current behaviour on wiki preview
    - [x] Fix current behaviour on wiki preview
        - [x] Rewrite all links and get preview links working
        - [x] Make sure all links are on-par with the wiki `show` page
        - [x] TDD `WikiLinkFilter` and get it working
        - [x] Hook `WikiLinkFilter` up
    - [x] Fix tests 
        - [x] Fix `markdown_spec`
        - [x] Fix `wiki` spinach feature
        - [x] Wait for [build](https://gitlab.com/gitlab-org/gitlab-ce/commit/4f50dd2/builds) to pass
        - [x] Make sure all wiki-related pages are working as expected (history, all pages, etc.)
    - [x] Test in different ruby versions
    - [x] GitLab instances hosted on a relative URL
    - [x] Non-markdown rendering formats?
        - [x] RDoc
        - [x] ASCIIDoc
        - [x] Create issues to fix things for RDoc and ASCIIDoc
    - [x] Gauge performance impact
    - [x] Refactor
    - [x] Re-organize commits
    - [x] Make sure [build](https://gitlab.com/gitlab-org/gitlab-ce/commit/f860e9a8dcabe7d5f160c32fc549807c98baa4a1/builds) passes
    - [x] Respond to @rymai's comments
        - [x] `class WikiLinkFilter < HTML::Pipeline::Filter`
        - [x] blank line after guard clause
        - [x] keyword arguments for `wiki` and `slug`
        - [x] invert the condition
        - [x] inline `user` in spec
        - [x] Make sure spec names are not out of date
        - [x] Comment for each rewrite rule
    - [x] Add CHANGELOG entry
    - [x] Reorganize commits
    - [x] Make sure [build](https://gitlab.com/gitlab-org/gitlab-ce/commit/19b91e749a6320d12fb299d33f1f6440777e0e26/builds) passes
    - [ ] Wait for merge

See merge request !4432

- Need to pass in a `page_slug` to the filter, so it can rewrite based
  on the current page (all links are rewritten to the level of the app root).
- The earlier `markdown_preview` endpoint was at the level of the wiki.
  We need to know the current page (for rewriting, as above), so this
  commit moves the endpoint to the level of a wiki page.
- Fix all tests

Original Comments
=================

- Linking behaves as per rules documented here:
  https://gitlab.com/gitlab-org/gitlab-ce/blob/16568-document-wiki-linking-behavior/doc/markdown/wiki.md
- All links (to other wiki pages) are rewritten to be at the level of
  the app root. We can't use links relative to the current
  page ('./foo', 'foo', '../foo'), because they won't work in the
  markdown preview, where the current page is suffixed with `/edit`
- Move existing `WikiLinkFilter` specs to `WikiPipeline` spec. It makes
  sense to run these tests on the combined output of the pipeline,
  rather than a single filter, since we can catch issues with
  conflicting filters.
- Add more tests to cover the new linking

@rymai's Review
===============

- Classes nested under `WikiLinkFilter` should declare `WikiLinkFilter`'s
  inherit, so nothing changes if the nested class is loaded first.
- Add a blank line after a guard clause
- Use keyword arguments for the `Rewriter` constructor
- Invert a condition - use `if` instead of `unless`
- Inline a `let` in `WikiPipeline` spec - it was only used in a single place
- Change out of date spec names
- Add a comment for every rewrite rule in `Rewriter`

Shows award emoji for comments to all users who are logged in.

## What does this MR do?
Fixes the award emoji picker not showing up for users who weren't able to edit another user's comment.

## Screenshots (if relevant)
Before:

![Screen_Shot_2016-06-08_at_5.18.04_PM](/uploads/a8b40ba319481f53d55de055d84649b9/Screen_Shot_2016-06-08_at_5.18.04_PM.png)

After (logged in):
![Screen_Shot_2016-06-08_at_5.16.16_PM](/uploads/535edcca6fbdd8834beda39399f85384/Screen_Shot_2016-06-08_at_5.16.16_PM.png)

After (logged out):
![Screen_Shot_2016-06-08_at_5.15.08_PM](/uploads/7f0dff751714c7d20f78b56893cacd59/Screen_Shot_2016-06-08_at_5.15.08_PM.png)

cc: @fatihacet 

See merge request !4547

Show loading indicator for autocomplete

![autocomplete](/uploads/397df34689aeb5f4cbca99aea209c8cf/autocomplete.gif)

Closes #15435

See merge request !4182

#17284 Edit form does not cancel correctly

## What does this MR do?

Replaces the markdown textarea with the currently saved note text when the note edit is cancelled.

## Are there points in the code the reviewer needs to double check?

No

## Why was this MR needed?

Allows the user to revert back to the original note text, as expected, using cancel.

## What are the relevant issue numbers?

Fixes #17284.

## Screenshots

![17284](/uploads/c3598de6f8e70d52275ef25907d92d42/17284.gif)

See merge request !4175

Side nav scrolls if content height taller than screen

## What does this MR do?
Header logo is now fixed at top; put back scrolling functionality in side nav

## Screenshots (if relevant)
<img src="/uploads/e1d1b60c296c39c61d7fe7da22b6c52d/scroll.gif" width="600px">

See merge request !4513

Disable CSRF protection when serving Git HTTP clients

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/18331

See merge request !4538

Add new icons in left sidebar and layout nav

Closes #17609    
Closes #17240    
Closes #17321    

![Screen_Shot_2016-06-02_at_8.47.38_AM](/uploads/f254fe81cba65cbdeb8e5720f0b2fc19/Screen_Shot_2016-06-02_at_8.47.38_AM.png)
![Screen_Shot_2016-06-02_at_8.53.33_AM](/uploads/2c27284f0a4916fe447794d9b663c4df/Screen_Shot_2016-06-02_at_8.53.33_AM.png)

@skyruler I couldn't find the icons for `Projects`, `Merge Requests`, `Snippets`, or `Help` in the design repo. Do you have them available somewhere?

See merge request !4268

Use container helper instead of hard coded container

## What does this MR do?
Add `container_class` helper class to fix layout bug

## What are the relevant issue numbers?
#18333 
## Screenshots (if relevant)
Fixed:    
![Screen_Shot_2016-06-08_at_2.03.38_PM](/uploads/6e7575d711efb8eaf6ba641907b2d607/Screen_Shot_2016-06-08_at_2.03.38_PM.png)
Fluid:     
![Screen_Shot_2016-06-08_at_2.03.56_PM](/uploads/33d3f73781ca9ed129f853abfe9feb4b/Screen_Shot_2016-06-08_at_2.03.56_PM.png)

See merge request !4543

Fixed dropdown link line-height

## What does this MR do?

Fixes line-height issue on dropdown menu links

## Why was this MR needed?

The line-height of some dropdown links were taking the parent, most noticeable the commit header

## Screenshots (if relevant)

Before:

![Screen_Shot_2016-06-08_at_12.26.13](/uploads/51d6bbb4d727da0e3f27d715d6556d4f/Screen_Shot_2016-06-08_at_12.26.13.png)

After:

![Screen_Shot_2016-06-08_at_12.25.03](/uploads/2dcbfaa67982500eef42cfe4247cd54d/Screen_Shot_2016-06-08_at_12.25.03.png)

See merge request !4531

Improve ui consistency

* Decline idea of using gray buttons actively in UI
* remove icons from some buttons that already has text
* standardizes margin between buttons 


First step in https://gitlab.com/gitlab-org/gitlab-ce/issues/18324 

See merge request !4533

Allow custom text on 'almost there' page

Add a new application setting, after_sign_up_text. This is text to be
rendered as Markdown and shown on the 'almost there' page after a user
signs up, but before they've confirmed their account.

Closes #14098.

See merge request !4098

Allow clones from /namespace/project

Allow `git clone https://host/namespace/project` to work, in addition to `git clone https://host/namespace/project.git`

Closes #13840.

See merge request !4530

Redirect /namespace/project/info/refs to
/namespace/project.git/info/refs, so `git clone` works on either
namespace/project.git or namespace/project.

Added minimum password length to password field

Add a placeholder to the password field in the register form with the minimum number of characters required. Also added a pattern to the field to stop it submitting if less than that.

Closes #17765

See merge request !4458

Set target="_blank" for external links

Closes #14553

See merge request !4526

Disable Rails/UniqBeforePluck rubocop cop

 Rails/UniqBeforePluck seems to have some bugs 

* https://github.com/bbatsov/rubocop/issues/3122
* https://github.com/bbatsov/rubocop/issues/3148

and we had some problems in EE with that https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/425#note_12245005

See merge request !4477

[ci skip]

Add a new application setting, after_sign_up_text. This is text to be
rendered as Markdown and shown on the 'almost there' page after a user
signs up, but before they've confirmed their account.

Tweak the styles for that page so that centered lists look reasonable.

Sidebar updates

## What does this MR do?
Updates `nav/_admin.html.haml` and `nav/_explore.html.haml` to match new compact sidebar design

## Screenshots (if relevant)
![Screen_Shot_2016-06-07_at_2.19.18_PM](/uploads/5699987001de901a67d503b097d1b19d/Screen_Shot_2016-06-07_at_2.19.18_PM.png)

See merge request !4517

Update Knapsack report only on master

## What does this MR do?

Makes to generate Knapsack report only on master. This speed-ups tests, because the last stage can be run significant time after previous stages.


See merge request !4540

Optimize issuable APIs interaction with notes

See merge request !4471

Remove notification settings in batches

## What does this MR do?

This improves the performance of the migration `db/migrate/20160603180330_remove_duplicated_notification_settings.rb` by removing duplicate rows in batches instead of using a single big `DELETE FROM` query.

## Why was this MR needed?

The original migration would locally take 45 minutes to complete, possibly up to hours on GitLab.com and similar setups.

## What are the relevant issue numbers?

#18289

See merge request !4529

Removable labels from filtered issuables label bar

When filtering by labels, a remove button appears next to each label. This then removes that label & refreshes the issuable filter form

![Screen_Shot_2016-05-17_at_12.07.47](/uploads/b21e86247aa1bbfd347bef1eb21f1562/Screen_Shot_2016-05-17_at_12.07.47.png)

Closes #15474

See merge request !4178

Backported from EE shared form for web hooks

## What does this MR do?

EE uses a shared form for web hooks - so this brings that across to CE.

See merge request !4457