Fixes to Notifications Window

The Avatars in the notifications window can overflow if the text next to it is short enough that there are only two lines of text. We need to clear the float set by the avatar to avoid this.

Also, it'd be nice to have a no_avatar placeholder for notifications by users with no avatars, so at least things look a little more consistent, layout wise. It may also be a good opportunity to use a better no_avatar image than the one provided by phpBB currently.

http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=44779