Add a "Change priority to: ..." Herald action

Summary:
Ref T7848. This is a companion to "Change status to: ...".

(I'm pretty sure the only reason I didn't originally write these was the tokenizer bug in D14682, I just forgot about it).

This is basically a copy/paste of the "status" action.

Test Plan:

• Wrote a rule to change task priorities.
• Edited a task.
• Saw rule fire properly.
• Tokens also stick around correctly.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T7848

Differential Revision: https://secure.phabricator.com/D14683