messages: Stop joining through every table.

Calling `.select_related()` with no arguments joins through every
possible table, recursively.  In this case, this currently produces a
query which joins through forty-three tables.

This is rather inefficient, particularly for what is a very common
call which should be very fast.

No callsite depends on having prefetched any joined table on the
object; drop all of the joins.

zerver/models/messages.py

@@ -622,9 +622,7 @@ def get_usermessage_by_message_id(
     user_profile: UserProfile, message_id: int
 ) -> Optional[UserMessage]:
     try:
-        return UserMessage.objects.select_related().get(
-            user_profile=user_profile, message_id=message_id
-        )
+        return UserMessage.objects.get(user_profile=user_profile, message_id=message_id)
     except UserMessage.DoesNotExist:
         return None