Clean up some Differential behaviors

Summary:
Ref T2222.

  - Restore mail tags for ApplicationTransactions mail.
  - Restore subject line verbs.
  - Denormalize line count and repository PHID.
  - Fix an issue with the mailgun adapter where headers weren't attached properly.

Test Plan: Sent some mail, verified it had correct subjects and tags.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8378