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.

test/controllers/rooms/directs_controller_test.rb

@@ -29,4 +29,34 @@ class Rooms::DirectsControllerTest < ActionDispatch::IntegrationTest
       assert_redirected_to root_url
     end
   end
+
+  test "destroy can't reach a closed room the member didn't create" do
+    sign_in :kevin
+
+    assert_no_difference -> { Room.count } do
+      delete rooms_direct_url(rooms(:designers))
+    end
+
+    assert rooms(:designers).reload.persisted?
+  end
+
+  test "destroy can't reach an open room the member didn't create" do
+    sign_in :kevin
+
+    assert_no_difference -> { Room.count } do
+      delete rooms_direct_url(rooms(:hq))
+    end
+
+    assert rooms(:hq).reload.persisted?
+  end
+
+  test "destroy can't reach a room the member isn't in at all" do
+    sign_in :jz
+
+    assert_no_difference -> { Room.count } do
+      delete rooms_direct_url(rooms(:david_and_kevin))
+    end
+
+    assert rooms(:david_and_kevin).reload.persisted?
+  end
 end