WIP: DISCUSSION adds Ldap group sync for new users

One of our major use cases for gitlab is its OAuth provider feature. Multiple internal services use it as kind of an Ldap-OAuth Gateway. However, since some versions (I believe pre 8.7 to now) the LDAP Group Sync does only sync users once every hour. There have been multiple issues and merge request regarding this issue, most notably !500 (closed). Please also see #730.

For us, it would be great if new users (as in 'just logged into Gitlab for the first time'), would have their groups / permission synced right away. It is common that new users are created on the fly by people using Gitlab to login our other services.

This MR is a naive approach to force a sync via the User #post_create_hook. Are we missing something crucial here?