Stop reads and writes to projectPHIDs property on ManiphestTask

Summary: Ref T5245. This property predates edges and is unusual in modern applications. Stop writes to it and populate it implicitly from edges when querying.

Test Plan:

• Viewed task list.
• Created a task.
• Added and removed projects from tasks.

Reviewers: joshuaspence, chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5245

Differential Revision: https://secure.phabricator.com/D9851