Share Revision/Task attaching code

Summary:
We have this code in two places; split it into an editor class so we can share
it.

This also fixes some probems with this field not detaching tasks properly.

Test Plan:

• Created a revision with no attached tasks.
• Attached it to a task.
• Updated it.
• Detached it.
• Used web UI to attach/detach tasks/revisions.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, btrahan, epriestley

Differential Revision: 1225