Add "Change Priority", "Open / Close", "Comment" and "Assign" actions to Maniphest batch editor

Summary: I think these are all the actions which make any sense.

Test Plan:

• Performed and verified each action through the batch editor.
• Performed a large batch edit which applied each action type multiple times and verified the aggregate behavior was correct.

Reviewers: btrahan, cpiro

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T441

Differential Revision: https://secure.phabricator.com/D1971