Add new-style transaction editor to Maniphest and switch priority edits to it

Summary: Ref T2217. All the reads route through new code already, start swapping writes over. This is the simplest writer, used when the user drag-and-drops stuff on the task list.

Test Plan: Dragged and dropped stuff across priorities. Got a transaction and some email. Verified the email and transaction looked OK, threaded properly, etc.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2217

Differential Revision: https://secure.phabricator.com/D7080