On request edit, clear conflict tags and recheck

If editing a push request, clear any conflict tags that may have been set.

If the edited push request is already pickme'd for a push, also requeue that
pickme to be re-checked for conflicts.
Tests for clearing tags, requeueing
Merge pull request #70 from rschlaikjer/conflict-check-on-edit

Clear any conflict tags that may have been set when a request is editted.

If the edited push request is already pickme'd for a push, also requeue that
pickme to be re-checked for conflicts.