Add git conflict detection

Add methods to automatically check for merge conflicts between:
- Pickme'd branches and master
- Pickme'd branches and other pickme'd branches for the same push

Whenever a request is pickmed, a git action is queued that:
- Updates the local copy of master to current head
- Clones or updates the local copy of devs/$dev branch
- Creates a temporary test branch off master
- Attempts to merge the pickmed branch onto the test. If this fails, it marks the pickme as conflict-master
- If it does not conflict with master, it then applies every other pickme in the push on top of the test branch as well, checks if it works, and then reverts. 

All the pickmes that fail to merge are included in a 'conflicts' section on the pickme on pushmanager