Add a "USERS" section to audit emails listing commit authors and committers

Summary: Fixes T5872. This won't show up in the initial email until T4896 is further along.

Test Plan:
```
RECIPIENTS
  discoball (Disco Ball)

BODY
epriestley added a comment.

ffkn

USERS
  epriestley (Author)

COMMIT
  http://local.aphront.com:8080/rPOEMS165b6c54f487
```

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5872

Differential Revision: https://secure.phabricator.com/D10266