Make "reject" and "blocking reviewer" block acceptance in Differential

Summary:
Ref T1279. This is a logical change.

  - "Reject" (nee "Request Changes") is now sticky. The review won't transition to "Accepted" until the reviewer clears their objection. In practice, I think it always worked like this anyway (without technical enforcement, users just followed this rule naturally, since disobeying this rule is kind of a dick move) so I don't expect this to change much. I think this rule is easier to understand than the old rule now, given the multi-reviewer status and blocking reviewers.
  - "Blocking Reviewer" and "Reject" now prevent a revision from transitioning to "Accepted". When reviewers accept, resign, or are removed, we do a check to see if the reivsion has: at least one user reviewer who has accepted; zero rejects; and zero blocks. If all conditions are satisfied, we transition it to "accepted".

Practically, the primary net effect of this is just to make blocking reviews actually block.

This is pretty messy, but there's not much we can do about it until after T2222, since we have two completely separate editor pathways which are both responsible for adjusting status. Eventually, these can merge into a single sane editor which implements reasonable rules in reaonable ways. But that day is not today.

Test Plan: With three users and a project, made a bunch of accepts, rejects, resigns and reviewer removals. I think I probably covered most of the pathways? There are a lot of interactions here.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, wisutsak.jaisue.7

Maniphest Tasks: T1279

Differential Revision: https://secure.phabricator.com/D7245