Use whoami instead of $USER

Set OmniAuth full_host parameter to ensure redirect URIs are correct

### What does this MR do?

This MR sets the OmniAuth `full_host` parameter to the configured GitLab URL to ensure the `redirect_uri` parameter is called with the right GitLab host.

### Why was this MR needed?

[OmniAuth attempts to grab the request URI](http://awesomeprogrammer.com/blog/2012/12/09/dealing-with-omniauth-redirect-uri-mismatch-invalid-port-number-gotcha/) and use that. If you set up a reverse proxy that terminates SSL at the Web server layer (e.g. https://gitlab.domain.com), omniauth will use the internal URL (e.g. http://my-host:8080) in its redirect URI unless all the Web server headers are properly set (e.g. `X-Forwarded-Port`, etc.). This is easy to forget or mess up, and it's better to ensure that OmniAuth has the right value from the start.

### What are the relevant issue numbers?

Closes #1967

See merge request !991

Fix label read access for unauthenticated users

The label page was added to navigation for unauthorized users because
the previously used milestone read permission was still checked. This
has been fixed and read access to labels is now granted (again) for
public projects.

This regression has been introduced in
07efb17e10fe26a01b60d8441868f9fbda0768f2 (7.12).

See also 9bcd36396b9b71467f66dd4ed79ab709bb5d027a.

Refs !836, !842.

See merge request !1000

Dynamically check `:admin_*` ability in issuable context

Closes #2001

See merge request !1002

Closes #2001

The label page was added to navigation for unauthorized users because
the previously used milestone read permission was still checked. This
has been fixed and read access to labels is now granted (again) for
public projects.

This regression has been introduced in
07efb17e10fe26a01b60d8441868f9fbda0768f2 (7.12).

See also 9bcd36396b9b71467f66dd4ed79ab709bb5d027a.

Refs !836, !842.

Fix bug where notes were being rendered with wrong color preferences due to caching

### What does this MR do?

This MR adds the user's color preferences as part of the caching key for notes.

### Why was this MR needed?

This should fix a bug introduced via 4d80360b where notes were being loaded from the cache without regard to the current user's color scheme.

### What are the relevant issue numbers?

Closes #2008

See merge request !994

Fix OAuth provider bug where GitLab would not go return to the redirect_uri after sign-in

### What does this MR do?

This MR fixes a bug when GitLab is used as an OAuth provider (e.g. to GitLab CI) where the user is not redirected back to the original URI after a successful sign-in. This MR tweaks the Doorkeeper configuration to set the `user_return_to` Devise session variable, as mentioned in this [Stack Overflow](http://stackoverflow.com/questions/14504739/how-to-use-both-devise-and-doorkeeper-gems) post.

### Why was this MR needed?

If you attempt to "Login via GitLab" from GitLab CI and aren't logged out completely, GitLab asks for your username and password. After you login, you get directed to the GitLab dashboard instead of back to GitLab CI. It's easy to reproduce:

1. Set up a GitLab CI and GitLab instance (e.g. ci.gitlab.com, gitlab.com).
2. Start an Incognito Window so that you are logged out of GitLab CI and GitLab.
3. Go to the GitLab CI instance. Click the "Login with GitLab".
4. Enter in your credentials.
5. Watch your browser login to GitLab and not return to GitLab CI.

### What are the relevant issue numbers?

Closes #1612

See merge request !998