Patch clean_prefix for new text editor

The new editor seems to force mentions to have ! even without a nick.

discord/ext/commands/help.py

@@ -344,7 +344,8 @@ class HelpCommand:
         # consider this to be an *incredibly* strange use case. I'd rather go
         # for this common use case rather than waste performance for the
         # odd one.
-        return self.context.prefix.replace(user.mention, '@' + user.display_name)
+        pattern = re.compile(r"<@!?%s>" % user.id)
+        return pattern.sub("@%s" % user.display_name, self.context.prefix)
 
     @property
     def invoked_with(self):