(stable) 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 (bye k thx)
• Create another room, myself only
• Join room from second account
• See ability to only remove myself
• Remove myself
• Rejoin
• Add third account
• Log into first account
• Boot off randos
• Test joining by green button, message, and by + sign.

Policy consistency:

• As a user who can not edit the room, tried to add other members. Received policy exception. The + button is currently visible and enabled for all users (even users who have not joined the room) but this is pre-existing.

Reviewers: chad

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D17696