Merge branch 'set-omniauth-full-host' into 'master'

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

CHANGELOG

@@ -4,6 +4,7 @@ v 7.14.0 (unreleased)
   - Fix label read access for unauthenticated users (Daniel Gerhardt)
   - Fix OAuth provider bug where GitLab would not go return to the redirect_uri after sign-in (Stan Hu)
   - Fix file upload dialog for comment editing (Daniel Gerhardt)
+  - Set OmniAuth full_host parameter to ensure redirect URIs are correct (Stan Hu)
   - Expire Rails cache entries after two weeks to prevent endless Redis growth
   - Add support for destroying project milestones (Stan Hu)
 

config/initializers/7_omniauth.rb

@@ -11,6 +11,7 @@ if Gitlab::LDAP::Config.enabled?
   end
 end
 
+OmniAuth.config.full_host = Settings.gitlab['url']
 OmniAuth.config.allowed_request_methods = [:post]
 #In case of auto sign-in, the GET method is used (users don't get to click on a button)
 OmniAuth.config.allowed_request_methods << :get if Gitlab.config.omniauth.auto_sign_in_with_provider.present?