Record build success or failure on buildable objects

Summary:
Fixes T4810. When a buildable completes, make an effort to update the corresponding object with a success or failure message. Commits don't support this yet, but revisions do.

{F144614}

Test Plan:
  - Used `bin/harbormaster build` and `bin/harbormaster update` to run a pile of builds.
  - Tried good/bad builds.
  - Sent some normal mail to make sure the mail reentrancy change didn't break stuff.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4810

Differential Revision: https://secure.phabricator.com/D8803