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