Treat "request review" more like an update

Summary:
After "reject; plan changes; request review", revisions go back to "needs revision". Instead, they should remain in "needs review" (the reviewers need to review comments on the "request review", in the normal case). Generally, "request reivew" should act a lot like "update", just not actually change the diff.

To accomplish this, downgrade reviewers on "request review" to "rejected older", just like we would on an update.

Test Plan: Did "reject; plan; request", revision ended in "needs review". Rejected it into "needs revision"; updated it into "needs review".

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: dctrwatson, epriestley

Differential Revision: https://secure.phabricator.com/D8558