Disallow "Accept" on drafts, allow "Resign"

Summary:
Depends on D18801. Ref T2543. See PHI229. I missed "Accept" before, but intended to disallow it (like "Reject") since I don't want drafts to be reviewable.

However, "Resign" seems fine to allow? So let's allow that for now.

Test Plan: Was no longer offered "Accept" on draft revisions.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T2543

Differential Revision: https://secure.phabricator.com/D18802