Herald support for changing task status

Summary:
Ref T7848. This patch is incomplete and has the following issues:

• Multiple statuses can be entered on the edit rule page (only the first one is used).
• Statuses are not rendered correctly when re-editing a rule.

Test Plan: Applied to our local phab instance and verified it works with our task workflow.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: joshuaspence, revi, epriestley, jsmith

Maniphest Tasks: T7848

Differential Revision: https://secure.phabricator.com/D14359