R1:c6add6ae7329

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…