Conpherence - implement join / view rules for rooms

Summary:
Ref T7585. This implements everything specified, with a few caveats

• since rooms you have yet to join can't be viewed in the column yet, the column view has some bugs and isn't expected to work.
• the room you're looking at is just pre-pending to the top of the "recent" list

Test Plan: made a room that no one could join. verified when viewing that there was no comment ui. made a room that others could join. verified folks who had yet to join had a "join" button with an area for text. tried joining with / without message text and it worked in both cases

Reviewers: chad, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7585

Differential Revision: https://secure.phabricator.com/D12149