R1:42d49be47bfc

Change Differential revision buckets to focus on "next required action"

Summary:
Ref T10939. Ref T4144. This splits the existing buckets ("Blocking Others", "Action Required", "Waiting on Others") into 6-7 buckets with a stronger focus on what the next action you need to take is.

See T10939#175423 for some discussion.

Overall, I think some of the root problems here are caused by reviewer laziness and shotgun review workflows (where a ton of people get automatically added to everything, probably…