EditedMessage should use 'username' as 'user' is not present in slack…

… API response (#2021)

* EditedMessage should use 'username' as 'user' is not present in slack API response

* We will keep the existing user but will also try username if it fails

---------

Co-authored-by: Rodrigo Albuquerque <ralbuquerque@apple.com>
Co-authored-by: Jacob Tomlinson <jacob@tomlinson.email>

opsdroid/connector/slack/create_events.py

@@ -101,7 +101,7 @@ async def edit_message(self, event, channel):
         return events.EditedMessage(
             text,
             user=user_name,
-            user_id=event["message"]["user"],
+            user_id=event["message"].get("user", event["message"]["username"]),
             target=event["channel"],
             connector=self.connector,
             event_id=event["ts"],