Add re-bridge command

[surakin]

This is probably low priority but I think it would be useful to have a command to 're-bridge' a room, that would work like a room upgrade. This would allow bridge admins to change the default room bridge settings without having to delete a room portal and create it again relying only on backfilling to preserve the room history.

(I'm adding this here because several bridges could benefit from it)

(personally I'd like to use it to disable encryption on a few bridged rooms with years of history, but there may be other use cases)