Fix an issue where "Request Review" of a fully-accepted revision would transition to "Accepted"

Summary:
Ref T10967. This is explained in more detail in T10967#217125

When an author does "Request Review" on an accepted revision, void (in the sense of "cancel out", like a bank check) any "accepted" reviewers on the current diff.

Test Plan:

• Create a revision with author A and reviewer B.
• Accept as B.
• "Request Review" as A.
• (With sticky accepts enabled.)
• Before patch: revision swithced back to "accepted".
• After patch: the earlier review is "voided" by te "Request Review", and the revision switches to "Review Requested".

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10967

Differential Revision: https://secure.phabricator.com/D17566