events: Pass a realm object into send_event.

This is a preparator refactor for supporting hosting different Tornado
processes on different servers; to look up which Tornado server we
should be sending the event to, we'll need the realm object.

docs/tutorials/writing-views.md

@@ -283,7 +283,7 @@ def do_set_realm_name(realm, name):
         property='name',
         value=name,
     )
-    send_event(event, active_user_ids(realm))
+    send_event(realm, event, active_user_ids(realm))
 ```
 
 `realm.save()` actually saves the changes to the realm to the