Fixed up the coding for the BBCode [mention][/mention] that can be used instead of the simple_mention extension version.
Creates a @mention link to the named user's profile.
Can be used either on its own or as a drop-in replacement for the default BBCode used by the Simple Mentions extension.
Note that the _baseurl parameter is not intended to be user-configurable; it's just a way of grabbing the forum's relative path if it's not the root URL. Worst case scenario if a user deliberately changes it is that you get a broken (but still internal) link.