R1:c1e8b394ccfe

Fix join and remove policy checks for Conpherence

Summary:
I think these got munged when I removed CAN_JOIN.

- If you can view the room, you can join it.
- ~~If you can view the room, you can add others to it.~~ This rule adjustment was removed, see discussion on the revision.
- If you are a participant in the room, you can remove yourself.
- If you can edit a room, you can remove anyone.

Test Plan:
Normal feature set:

- Create a new room that only I can edit, viewable by all users.
- Leave room…