Fix an issue where rejecting reviewers weren't powerful enough

Summary:
Previously, "reject" and "reject older" were separate statuses. Now, they're both shades of "reject".

Set the "older reject" flag properly when we find a non-current reject.

Test Plan:

• User A accepts a revision.
• User B rejects it.
• Author updates it.
• Before patch: incorrectly transitions to "accepted" ("older" reject is ignored).
• After patch: correctly transitions to "needs review".

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D17653