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