Add ability to @mention specific users in posts

http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=42807

It would be nice to have a @mention feature, akin to what Twitter, Github, Facebook et al. have. The behaviour I had in mind is this: you start typing @someusername and a small window pops up showing possible usernames (maybe up to 10). When you submit the post, the username is linked to the user's profile.

To sum up:

• A new [mention][/mention] bbcode
• A new popup-up when you start writing @ after a space if you are in WYSIWYG mode
• @mention parsed automatically when the post is submitted (with a checkbox and an option to disable it, like for the smileys)