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/opens_controller_test.rb

@@ -57,4 +57,23 @@ class Rooms::OpensControllerTest < ActionDispatch::IntegrationTest
     put rooms_open_url(rooms(:designers)), params: { room: { name: "Doesn't matter" } }
     assert_equal rooms(:designers).memberships.count, User.count
   end
+
+  test "a direct room can't be promoted to open by its creator" do
+    sign_in :kevin
+    direct = rooms(:bender_and_kevin)
+
+    put rooms_open_url(direct), params: { room: { name: "Watercooler" } }
+
+    assert_equal "Rooms::Direct", Room.find(direct.id).type
+    assert_equal [ users(:bender).id, users(:kevin).id ].sort, Room.find(direct.id).user_ids.sort
+  end
+
+  test "a direct room can't be promoted to open by an administrator either" do
+    direct = rooms(:david_and_kevin)
+
+    put rooms_open_url(direct), params: { room: { name: "Watercooler" } }
+
+    assert_equal "Rooms::Direct", Room.find(direct.id).type
+    assert_equal [ users(:david).id, users(:kevin).id ].sort, Room.find(direct.id).user_ids.sort
+  end
 end