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