Make Maniphest project edits more transaction-oriented

Summary: Ref T5245. Currently, task/project links rely on side effects in `save()`. Make them more transaction-oriented, with the goal of moving fully to edges a few diffs down the line.

Test Plan:
  - Added and removed projects using "Edit Task", "Associate Projects" comment action, and Herald.
  - Verified database ended up in the expected state.

Reviewers: chad, btrahan, joshuaspence

Reviewed By: joshuaspence

Subscribers: epriestley

Maniphest Tasks: T5245

Differential Revision: https://secure.phabricator.com/D9833