Allow Admins to remove the Login with buttons for OAuth services and still be able to import on the changelog

On Application Settings Page let the user select the enabled OAuth Sign in sources instead of the disabled ones

if omniauth is enabled and if there are button_based_providers available show the disable OAuth sources

stub Devise.omniauth_providers to return GitHub even if the gitlab.yml has no omniauth provider enabled
This will fix failing tests in case gitlab.yml file has no omniauth providers enabled

Update 8.8 guides

See merge request !4119

Relative Links in the Wiki Are Broken

- [ ] #16568 (!4050) Relative links in wiki are broken
    - [x] Investigate issue

  
    - [x] Implementation / Fix

        - [x] Write (failing) tests for `WikiLinkFilter`
        - [x] Link to `./bar` should either get rewritten correctly or left alone
        - [x] Link to `./bar.md` should maybe get rewritten correctly (is left alone currently)
        - [x] Link to `bar.md` should get rewritten correctly


            - [x] Check if this indeed a bug

    - [x] Make sure CI is green
    - [x] Assign to endboss

    - [x] Wait for review

    - [x] Implement review feedback
    - [ ] Wait for merge


See merge request !4050

- Separate 'exercise' and 'verify' steps of tests.
- Use `build_stubbed` instead of `build`

- And fix behavior for non-file hierarchical links.

Add snippet tab under user profile

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/15572

See merge request !4001

Remove unused `icon_for_event` helper

Extracted from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4090

See merge request !4120

Don't show the Todo count if there aren't any Todos.

When there are 0 Todos, the Todos icon still had the blue badge with a "0" in it, which made me think I had a new Todo left. This makes it less annoying.

Follow-up on !4077.

**Before:**

![Screen_Shot_2016-05-10_at_9.36.57_PM](/uploads/e0dfafae01692286096606c3251f0997/Screen_Shot_2016-05-10_at_9.36.57_PM.png)

**After:**

![Screen_Shot_2016-05-10_at_10.01.02_PM](/uploads/30a8bafe8f54862418eb4045e5777a9a/Screen_Shot_2016-05-10_at_10.01.02_PM.png)

cc: @jschatz1 @rspeicher 

See merge request !4113

Improve documentation and web test for web hooks

Tips and documentation of actual hook behavior. Improved user feedback
when testing hooks via the web UI.

See merge request !4015

Remove Rails monkey-patches now that we're using Rails 4.2.6

Closes #17356.

See merge request !4115

Improve log message when a project is destroyed to include the namespace

See merge request !4114

Restrict starred projects to viewable ones

`User#starred_projects` doesn't perform any visibility checks. This has
a couple of problems:

1. It assumes a user can always view all of their starred projects in
   perpetuity (project not changed to private, access revoked, etc.).
2. It assumes that we'll only ever allow a user to star a project they
   can view. This is currently the case, but bugs happen.

Add `User#viewable_starred_projects` to filter the starred projects by
those the user either has explicit access to, or are public or
internal. Then use that in all places where we list the user's starred
projects.

Closes #17249.

See merge request !4108

Only generate repository push email once

The repository push email can be very expensive to generate, especially
with syntax-highlighted diffs. Instead of generating the email for each
recipient, generate one email object and reset the Message-Id and To
headers for each recipient. (Cloning would also be expensive in the case
of large emails, although probably not as bad as generating from
scratch.)

Closes #17270.

See merge request !4070