Add workboard trigger rule for changing task priority

Summary: This is a copy/paste/find-and-replace-all of the status rule added by D20288.

Test Plan: Made some triggers, moved some tasks, edited some triggers. Grepped for the word "status" in the new file.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D20325