MergeRequest#show performance improvements

This is a first pass on improving the performance of the `MergeRequests#show` page. Notable changes:

- The "Commits" tab is loaded lazily, so the initial page load should be much faster for MRs with many commits.
- Relative timestamps via `timeago` are only initialized once per load instead of `O(n^2)`. This greatly improves frontend rendering times for a large number of commits.
- Refactored `User.find_for_commit` to use a single ARel-generated SQL query instead of the old method which resulted in one query, and could result in up to three.

See merge request !838

Update Code of Conduct to Contributor Covenant 1.1.0

Make snippet filename optional.

Fixes #2384.

See merge request !1866

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

Fixes #2384.

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.

Starting shell document to GitLab basics

I added a new branch for all the changes on this MR https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/1863

Everything should look nicer now :)

See merge request !1869

Define 'everyone'

See merge request !1872

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