Make PhabricatorRepositoryCommmit schema changes for audit

Summary:

• Add a proper mailKey field to make these things mailable. Backfill all

existing objects.

• Denormalize authorPHID to the commit object so we can query by it

efficiently in a future diff. We currently use the search engine to drive
"commits by author" but that's not so good for audit, which needs more
constraints.

• Add an overall audit status field so we can efficiently query "commits that

needs your attention".

• Add enough code to convince myself that these fields are basically

reasonable and work correctly.

Test Plan:

• Ran schema upgrades. Checked database state afterward.
• Ran "reparse.php --owners --herald" to verify worker changes.
• Looked at a commit, altered aggregate status via audits / reparse.php,

verified it responded correctly.

Reviewers: btrahan, jungejason

Reviewed By: jungejason

CC: aran, epriestley, nh

Maniphest Tasks: T904

Differential Revision: https://secure.phabricator.com/D1706