R1:20d8b1bdd36a

Implement PhabricatorProjectInterface on ManiphestTask

Summary:
Ref T5245. This removes some hacks and activates two meaningful interactions:

- The "projects" field goes through shared code now.
- Mentioning projects in tasks using hashtags now tags them.

Test Plan:
- Viewed a task with projects.
- Viewed a task with no projects.
- Viewed a task with projects and board positions.
- Viewed a revision with projects.
- Made a `#hashtag` comment in Maniphest and got a project association.