[Refactor] Move Participants inside Event

Summary:
This is most of the code for the refactor, with some caveats:
1. This is somehow working. I tested it basically: synced a calendar, created and update event participants. I haven't run the system tests yet
2. There's still some flake8 style issues
3. I haven't written a migration yet.

Still, many bothans died to fix this code so I thought it would be nice to get something reviewable asap.

Test Plan: Manual tests.

Reviewers: kav-ya, emfree

Reviewed By: kav-ya, emfree

Subscribers: spang

Projects: #calendars

Maniphest Tasks: T631

Differential Revision: https://review.inboxapp.com/D865