Fix possible edge case when bot has nickname and is mentioned

Its valid for a user mention to be sent and parsed when a nickname
mention is expected. As such we should filter both from the message
contents to avoid message processing issues.

disco/bot/bot.py

@@ -267,7 +267,10 @@ class Bot(LoggingClass):
                 if msg.guild:
                     member = msg.guild.get_member(self.client.state.me)
                     if member:
-                        content = content.replace(member.mention, '', 1)
+                        # If nickname is set, filter both the normal and nick mentions
+                        if member.nick:
+                            content = content.replace(member.mention, '', 1)
+                        content = content.replace(member.user.mention, '', 1)
                 else:
                     content = content.replace(self.client.state.me.mention, '', 1)
             elif mention_everyone: