Maniphest - remove references to deprecated transaction type TYPE_PROJECTS from code

Summary:
...except the transaction class itself, which still needs some knowledge of these transactions for older installs.

Ref T5245. T5604 and T5245 are now in a similar place -- there's an unknown set of bugs introduced from my changes and there's still old display code lying around with some old transactions in the database. I'll stomp out the bugs if / when they surface and data migration is up next.

This revision also adds a "TransactionPreviewString" method to the edge objects so that we can have a prettier "Bob edited associated projects." preview of this transaction.

Test Plan: added a project from task detail and saw correct preview throughout process with correct project added. bulk removed a project from some tasks. added a project from the edit details pane.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T5245

Differential Revision: https://secure.phabricator.com/D11013