Remove CAN_JOIN policy from Conpherence

Summary: Fixes T12178, Fixes T11704 Not sure this feature gets any use and I can't find a similar option in other software, so removing it I think simiplifies a number of things. Removes CAN_JOIN and joinable is basically now CAN_VIEW and !$participating. Also removed some old transaction strings for other policies. Don't seem used.

Test Plan: Create a new room, edit room policies, see changes. Log into second account, search for rooms, everything now is visible.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12178, T11704

Differential Revision: https://secure.phabricator.com/D17675