Don't process /send requests for users who have hit their ratelimit (#13134)

changelog.d/13134.misc

@@ -0,0 +1 @@
+Apply ratelimiting earlier in processing of /send request.

synapse/handlers/message.py

@@ -903,6 +903,9 @@ class EventCreationHandler:
             await self.clock.sleep(random.randint(1, 10))
             raise ShadowBanError()
 
+        if ratelimit:
+            await self.request_ratelimiter.ratelimit(requester, update=False)
+
         # We limit the number of concurrent event sends in a room so that we
         # don't fork the DAG too much. If we don't limit then we can end up in
         # a situation where event persistence can't keep up, causing