Use DifferentialCommitMessageParser and Modular Transactions to implement "Auditors: ..."

Summary:
Ref T10978. Updates how we implement "Auditors: ..." in commit messages:

• Use the same parsing code as everything else.
  • (Also: parse package names.)
• Use the new transaction code.

Also, fix some UI strings.

Test Plan: Used bin/repository reparse --herald <commit> to re-run this code on commits with various messages (valid Auditors, invalid Auditors, no Auditors). Saw appropriate auditors added in the UI.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10978

Differential Revision: https://secure.phabricator.com/D17262