Scope room lookup to the type each controller administers

Rooms::DirectsController relaxes ensure_can_administer to true, because every
participant in a direct room may administer it. set_room was inherited unscoped,
though, so that relaxation applied to any room the caller was merely a member of:
DELETE /rooms/directs/<id> destroyed open and closed rooms and all their messages.

The same unscoped lookup let a direct room be loaded by the opens and closeds
controllers, where force_room_type promoted it. Promoting a DM to open grants every
user on the account membership and republishes the whole conversation, including the
other participant's messages; converting it to closed lets the initiator revise who
is in it and lock the other participant out.

Each controller now narrows room_scope to the types it may act on. Opens and closeds
keep reach into each other, since converting between them is a feature. Neither can
reach a direct room, and directs can only reach directs.

Room also refuses to change type away from Rooms::Direct, so the invariant holds for
any future caller of becomes! rather than only these two controllers.

app/controllers/rooms/opens_controller.rb

@@ -40,6 +40,12 @@ class Rooms::OpensController < RoomsController
       @room = @room.becomes!(Rooms::Open)
     end
 
+    # Open and closed rooms convert into each other, so both are in reach here. Direct
+    # rooms never are: promoting one would republish its history to the whole account.
+    def room_scope
+      Current.user.rooms.without_directs
+    end
+
     def broadcast_create_room(room)
       broadcast_prepend_to :rooms, target: :shared_rooms, partial: "users/sidebars/rooms/shared", locals: { room: room }
     end