Publish feed stories from Maniphest

Summary: I didn't get around to this earlier; add Feed/Maniphest integration.
This is partly motivated by wanting Projects to not be terrible. Pretty
straightforward.

Test Plan:

• Created, updated, reassigned and closed a task.
• Verified feed stories render reasonably.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, btrahan

Maniphest Tasks: T681

Differential Revision: 1232