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/models/room.rb

@@ -22,6 +22,8 @@ class Room < ApplicationRecord
 
   belongs_to :creator, class_name: "User", default: -> { Current.user }
 
+  validate :direct_rooms_keep_their_type, on: :update
+
   scope :opens,           -> { where(type: "Rooms::Open") }
   scope :closeds,         -> { where(type: "Rooms::Closed") }
   scope :directs,         -> { where(type: "Rooms::Direct") }
@@ -65,6 +67,15 @@ class Room < ApplicationRecord
   end
 
   private
+    # Open and closed rooms convert into each other freely. A direct room can't become
+    # either: its participants agreed to a private conversation, not to one whose
+    # audience someone else gets to widen afterwards.
+    def direct_rooms_keep_their_type
+      if type_changed? && type_was == "Rooms::Direct"
+        errors.add :type, "can't be changed for a direct room"
+      end
+    end
+
     def unread_memberships(message)
       memberships.visible.disconnected.where.not(user: message.creator).update_all(unread_at: message.created_at, updated_at: Time.current)
     end