Fix several issues with application interactions while importing commits

Summary:
  - Fixes T5851. Currently, if a commit has `Fixes T123`, we generate an email with just that before generating the commit email. Don't send/publish transactions about a commit before it imports (this is a tiny bit hacky, but well-contained and I don't think it causes any problems).
  - Fixes T4864. Currently, we try to parse Differential information even if Differential is not installed. Instead, do this only if Differential is installed.
  - Fixes T5771. Currently, if we can't figure out who the committer/author of a commit is, we don't publish a `Fixes T123` transaction. Instead, fall back to acting as "Diffusion" if we can't find a better actor. Most of this diff expands the role of application actors. The existing application actors (Herald and Harbormaster) seem to be working well.

Test Plan:
  - Pushed a commit with `Fixes T123` and verified it did not generate email directly. (The task half of the transaction still does, correctly.)
  - Uninstalled Differential and pushed a commit, got a clean import instead of an exception.
  - Commented out author/committer PHIDs and pushed stuff, saw a "Diffusion" actor.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5771, T4864, T5851

Differential Revision: https://secure.phabricator.com/D10221