R1:f22dc9d47a5d

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…