Add "Project tags added" and "Project tags removed" fields in Herald

Summary: Ref T13480. These fields don't serve a specific strong use case, but are broadly reasonable capabilities after "state" vs "change" actions were relaxed by T13283.

Test Plan: Wrote rules using the new fields, added and removed projects (and did neither) to fire them / not fire them. Inspected the transcripts to see the project PHIDs making it to the field values.

Maniphest Tasks: T13480

Differential Revision: https://secure.phabricator.com/D20946