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.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5245

Differential Revision: https://secure.phabricator.com/D10177