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

Closes #1612

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 7.14.0 (unreleased)
+  - 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)
   - Expire Rails cache entries after two weeks to prevent endless Redis growth
   - Add support for destroying project milestones (Stan Hu)

config/initializers/doorkeeper.rb

@@ -6,7 +6,8 @@ Doorkeeper.configure do
   # This block will be called to check whether the resource owner is authenticated or not.
   resource_owner_authenticator do
     # Put your resource owner authentication logic here.
-    # Example implementation:
+    # Ensure user is redirected to redirect_uri after login
+    session[:user_return_to] = request.fullpath
     current_user || redirect_to(new_user_session_url)
   end