Add participants ModularTransactions to Conpherence

Summary: Moves participants over to ModularTransactions, simplified a lot of the code. Fixes T12550

Test Plan:
Create a new room with just myself and myself + fake accounts.
Remove a person.
Remove myself.
Edit a room, topic.
Type some messages.
???
Profit

Reviewers: chad

Reviewed By: chad

Subscribers: Korvin

Maniphest Tasks: T12550

Differential Revision: https://secure.phabricator.com/D17685