Disable changing of target branch in new merge request page when a branch has already been specified

### What does this MR do?

This MR prevents a user from changing the target branch inside a new merge request page if the target branch has already been specified (e.g. from the shortcut in the activity feed). A link to "Change branches" is provided to allow the user to do this properly.

### Why was this MR needed?

If a user attempted to create a merge request from a shortcut with a pre-specified target branch (e.g. `master`), the dropdown would suggest that the user could change the branch. However, this did not actually work due to duplicate data elements (see #1830 for more details), and the target branch would always be set to the original, default branch. This would lead to confusion, potentially causing an improper merge request to be created.

### What are the relevant issue numbers?

Closes #1830

### Screenshot

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/eeec4f5d4b66625a14be41e5ecbdc016/image.png)

See merge request !846

Fix Error 500 when one user attempts to access another's personal, internal snippet

### What does this MR do?

This MR fixes an Error 500 that occurred if one user tried to access another's personal, internal snippet. Steps to reproduce:

### Why was this MR needed?

1. Go to `<hostname>/snippets/new`.
2. Select "Internal".
3. Create a snippet. Save the URL (e.g. `<hostname>/snippets/20`)
4. Logout and sign in as another user.
5. Go to the URL in step 3.

### What are the relevant issue numbers?

Closes #1815

See merge request !854

Include non-default ssh key location info

Fixes #875

Provide help for people who aren't SSH-clued by indicating
how to point to key files that have been named something
other than the default. A lot of people seem to assume that
the filename they choose is irrelevant and then wonder why
their private key is not being found by simple 'ssh' commands.

See merge request !507

Closes #1815

Disable changing of target branch in new merge request page when a branch has already been specified

Closes #1830

Refactor and spec line highlighting Javascript

- Replaces the fragile Spinach tests for this feature with Jasmine tests
- The "Don't scroll the page when we click a line number" behavior now works as expected

See merge request !820

Show user 2FA status on Admin::Users#show

| Enabled | Disabled |
| ------- | -------- |
| ![Screen_Shot_2015-06-19_at_2.58.42_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/b81f6a992366105c14e03852385b28f0/Screen_Shot_2015-06-19_at_2.58.42_PM.png) | ![Screen_Shot_2015-06-19_at_2.58.30_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/7bb8fd11b6c27615eef19154ffabe2de/Screen_Shot_2015-06-19_at_2.58.30_PM.png) |

Closes internal https://dev.gitlab.org/gitlab/gitlabhq/issues/2415

See merge request !851

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/875

Provide help for people who aren't SSH-clued by indicating
how to point to key files that have been named something
other than the default. A lot of people seem to assume that
the filename they choose is irrelevant and then wonder why
their private key is not being found by simple 'ssh' commands.

Update maintenance documentation to explain no need to recompile asssets for omnibus installations

See merge request !847

These are well covered by the new Jasmine tests, and faster!

Revert "Changed the placeholder text and modified the css class to make it visible"

This reverts commit c39c8234.

/cc @marin

See merge request !845

This reverts commit c39c8234.

Add init test for ProjectSearchResults

Regression test for: https://github.com/gitlabhq/gitlabhq/pull/9368

See merge request !788

See: https://github.com/gitlabhq/gitlabhq/pull/9368#issuecomment-110350335

Add a build preparation script, modify gitlab.ci.yml to use the script.

Updated the .gitlab-ci.yml to use a script which contains build preparation for runners which might run docker images with CI runner.
Regular builds should be not affected.

See merge request !834

Fix 403 Access Denied error messages when accessing Labels section in a project

This would occur if the project's issues or merge requests features were disabled. The change in 9bcd3639 caused `can?(current_user, :read_merge_request, project)` to be false if the merge request feature were disabled, so `authorize_labels!` needs to be changed accordingly.

Closes #1813

See merge request !836

Fix 403 Access Denied error messages when accessing Labels section in a project that has MRs disabled but issues enabled

Closes #1813

Fix behavior of ldap_person method in Gitlab::OAuth::User

Code tweaks in 45e9150a caused the ldap_person method to not return expected results.
Improved tests to cover the ldap_person method, which was previously stubbed.

Restored the previous implementation of ldap_person, as I could not find a more concise to write it that still works with all the test cases.

See merge request !837

Code tweaks in 45e9150a caused the ldap_person method to not return expected results.
Improved tests to cover the ldap_person method, which was previously stubbed.

Add 'proposed tweet' for RC1

See merge request !835