Add an "Auditors" field to commit messages which pushes audit requests when present

Summary:
Adds an optional "Auditors" field (like "Reviewers") to commit messages which gives installs a zero-config method for making audit requests.

This field does not appear on templates unless set, and is mostly ignored (but validated and preserved) by Differential.

It is then parsed by the daemons if present, and audit requests are pushed to valid users.

Test Plan: Made an "Auditors" commit and verified it was retained with "arc amend --show". Pushed it and verified the audit was triggered.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T904, T880

Differential Revision: https://secure.phabricator.com/D1793