Fix some EditEngine issues with rendering "invite" transactions

Summary:
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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9275

Differential Revision: https://secure.phabricator.com/D16284