Add an "Accepted Differential revision" field to Commit and pre-commit Content Herald rules
Summary: Refs T4195. Fixes T3936. You can't currently write rules like "block commits unless they're attached to an accepted revision"; allow that.
Test Plan: Pushed commits into a rule with this field, saw it work / not crash.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran, mbishopim3
Maniphest Tasks: T3936, T4195
Differential Revision: https://secure.phabricator.com/D7807