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.
Create a new room, edit room policies, see changes. Log into second account, search for rooms, everything now is visible.
Just in case, let's actually destroy the data in a separate change on a later date? I'm 99% sure this is safe but there is 0% chance we can roll this back if we destroy the data right now.
Let's not actually drop/remove the column for at least a week, just in case we get into trouble and want to revert this? We can't reasonably undo it after we DROP COLUMN since we lose all the data.
(Keep these too for now.)
And this one.