Route Maniphest editing and transactions primarily through shared code

Summary: Ref T418. Although Maniphest does not use ApplicationTransactions, we can fake a lot of it and provide a more uniform API. Deletes as much custom code from Maniphest as possible along the edit workflows, using core code instead.

Test Plan:
With custom fields:

  - Edited a task.
  - Created a task.
  - Queried a task with Maniphest.
  - Updated a task with Maniphest.
  - Used `?template=nnn` to create a similar task.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T418

Differential Revision: https://secure.phabricator.com/D7001