Stop writing new TYPE_PROJECTS transactions to Maniphest

Summary:
Ref T5245. We'll still display the old ones, but write real edge transactions now -- not TYPE_PROJECTS transactions.

Some code remains to show the existing transactions. The next diff will modernize the old transactions so we can remove this code.

Test Plan:
  - Previewed a project-editing comment.
  - Submitted a project-editing comment.
  - Edited a task's projects.
  - Batch edited a task's projects.

Reviewers: joshuaspence, chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5245

Differential Revision: https://secure.phabricator.com/D9852