R1:dd4537edef55

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.