Add a "Remove flag" action to Herald

Summary: Fixes T13409. This is a companion to the existing "Mark with flag" rule.

Test Plan: Used a "remove flag" rule on an object with no flag (not removed), the right type of flag (removed), and a different type of flag (not removed).

Maniphest Tasks: T13409

Differential Revision: https://secure.phabricator.com/D20796