Warn admin of granting admin rights during OAuth

Fixes #3951

Warning style the same as the warning a user sees on a archived project.



![Screenshot_from_2016-01-25_20-06-33](/uploads/42e6a218b963311fd20c0b7c87c8cf4f/Screenshot_from_2016-01-25_20-06-33.png)


See merge request !2601