Fix scope of "Accept" when you don't check all the "Force Accept" boxes

Summary:
Ref T12272. I wrote this correctly, then broke it by adding the simplification which treats "accept the defaults" as "accept everything".

This simplification lets us render "epriestley accepted this revision." instead of "epriestley accepted this revision onbehalf of: long, list, of, every, default, reviewer, they, have, authority, over." so it's a good thing, but make it only affect the reviewers it's supposed to affect.

Test Plan:

• Did an accept with a force-accept available but unchecked.
• Before patch: incorrectly accepted all possible reviewers.
• After patch: accepted only checked reviewers.
• Also checked the force-accept box, accepted, got a proper force-accept.

Reviewers: chad, lvital

Reviewed By: lvital

Maniphest Tasks: T12272

Differential Revision: https://secure.phabricator.com/D17634