Add trigger rule for adding projects to a task
Summary: Ref T13269. This is mostly copying code from the similar Herald implementation. Note that the drop effect preview always renders because we don't have the infrastructure to compare lists of edge targets.
Test Plan: Created some triggers, dragged some tasks around, checked that tasks that already had project membership didn't write additional edges.
Reviewed By: epriestley
Maniphest Tasks: T13269
Differential Revision: https://secure.phabricator.com/D20379