Add ability to remove the fork relationship from project settings

![Screen_Shot_2015-10-18_at_12.37.24](/uploads/676571642a4c90f7f286280d714599a3/Screen_Shot_2015-10-18_at_12.37.24.png)
![Screen_Shot_2015-10-18_at_12.37.28](/uploads/1a069ecfc4cd3b5438772a9c3f04b6fc/Screen_Shot_2015-10-18_at_12.37.28.png)

Replaces !1579.

Fixes #2578.

See merge request !1636