Fix some EditEngine issues with rendering "invite" transactions
Ref T9275. We were rendering too many transactions and/or over-rendering invitees.
Clean this logic up a bit:
- List all before/after invitees.
- Simplify the lists before rendering.
Test Plan: Viewed an event, edited invitees, got sensible human-readable transactions.
Reviewed By: chad
Maniphest Tasks: T9275
Differential Revision: https://secure.phabricator.com/D16284