New velocity macros for displaying user avatars with standard sizes

#largeUserAvatar($username) -> 120px, user profile
#mediumUserAvatar($username) -> 50px, primary avatar (comments, dashboard, annotations...)
#smallUserAvatar($username) -> 30px, secondary avatar (comment replies)
#resizedUserAvatar($username, $size) -> custom size in px, maximum for width and height
#useravatar($username) -> no resize, older macro