Send message after room has been shutdown

Currently the explanation message is sent to the abuse room before any
users are forced joined, which means it tends to get lost in the backlog
of joins.

So instead we send the message *after* we've forced joined everyone.

synapse/rest/client/v1/admin.py

@@ -488,17 +488,6 @@ class ShutdownRoomRestServlet(ClientV1RestServlet):
         )
         new_room_id = info["room_id"]
 
-        yield self.event_creation_handler.create_and_send_nonmember_event(
-            room_creator_requester,
-            {
-                "type": "m.room.message",
-                "content": {"body": message, "msgtype": "m.text"},
-                "room_id": new_room_id,
-                "sender": new_room_user_id,
-            },
-            ratelimit=False,
-        )
-
         requester_user_id = requester.user.to_string()
 
         logger.info("Shutting down room %r", room_id)
@@ -536,6 +525,17 @@ class ShutdownRoomRestServlet(ClientV1RestServlet):
 
             kicked_users.append(user_id)
 
+        yield self.event_creation_handler.create_and_send_nonmember_event(
+            room_creator_requester,
+            {
+                "type": "m.room.message",
+                "content": {"body": message, "msgtype": "m.text"},
+                "room_id": new_room_id,
+                "sender": new_room_user_id,
+            },
+            ratelimit=False,
+        )
+
         aliases_for_room = yield self.store.get_aliases_for_room(room_id)
 
         yield self.store.update_aliases_for_room(