Extract textual object list parsing from Differential

Summary:
Ref T2222. Currently, Differential has a fairly hairy piece of logic to parse object lists, like `Reviewers: alincoln, htaft`. Extract, generalize, and cover this.

  - Some of the logic can be simplified with modern ObjectQuery stuff.
  - Make `@username` the formal monogram for users.
  - Make `list@domain.com` the formal monogram for mailing lists.
  - Add test coverage.

Test Plan:
  - Ran unit tests.
  - Called `differential.parsecommitmessage` with a bunch of real-world inputs and got sensible results.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8445