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 unnecessarily), but these buckets haven't been updated since the introduction of blocking reviewers or project/package reviewers and I think splitting the 3 buckets into 6 buckets isn't unreasonable, even though it's kind of a lot of buckets and the root problem here is approximately "I want to ignore a bunch of stuff on my dashboard".

I didn't remove the old bucketing code yet since it's still in use on the default homepage.

This also isn't quite right until I fix the tokenizer to work properly, since it won't bucket project/package reviewers accurately.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4144, T10939

Differential Revision: https://secure.phabricator.com/D15924