Support 'User (mention)' style mentions used by recent client updates (#171)

* Support 'User (mention)' style mentions.

* Remove conditional that should no longer be needed.

(from PR feedback)

disco/bot/bot.py

@@ -313,8 +313,9 @@ class Bot(LoggingClass):
                 if msg.guild:
                     member = msg.guild.get_member(self.client.state.me)
                     if member:
-                        content = content.replace(member.user.mention_nickname, '', 1)
+                        # Filter both the normal and nick mentions
                         content = content.replace(member.user.mention, '', 1)
+                        content = content.replace(member.user.mention_nickname, '', 1)
                 else:
                     content = content.replace(self.client.state.me.mention, '', 1)
             elif mention_everyone: