Allows MR authors to have the source branch removed when merging the MR
closes#13191
The location of the checkbox might not be optimal so any feedback is welcome. Any other feedback too obviously.
Screenshot:
![Screenshot_2016-02-17_21.25.24](/uploads/a9c3eaafb39c6f5b4f0949a2278af6da/Screenshot_2016-02-17_21.25.24.png)
See merge request !2801