Currently deactivating a branch kinda leaves users in the dark, with
little way to know what has happened aside from inferring it from the
branch having disappeared from the main dashboard.
- surface the state of the branch in the PR dashboard (also surface
the target branch at all so users can see if their PR is targeted
as they expect as far as the mergebot is concerned)
- close & notify every PR to a branch being deactivated
- cancel any current staging to the branch (as a consequence of the
above)
Closes#632
