Don't downgrade accepts on update (fix "sticky accept")

Summary:
Fixes T12496. Sticky accept was accidentally impacted by the "void" changes in D17566.

Instead, don't always downgrade all accepts/rejects: on update, we only want to downgrade accepts.

Test Plan:

• With sticky accept off, updated an accepted revision: new state is "needs review".
• With sticky accept on, updated an accepted revision: new state is "accepted" (sticky accept working correctly).
• Did "reject" + "request review" to make sure that still works, worked fine.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12496

Differential Revision: https://secure.phabricator.com/D17605