[ticket/11525] Only remove group or user prefix from given avatar data

Until now, the user data had both user_id and group_id keys in the avatar
data. As both group_ and user_ prefixes were removed the group_id was
collapsed onto the user_id and therefore all users in the same group had
the same prefix for their uploaded avatars. This patch will make sure
that the correct id is used depending on whether it's a group's or user's
avatar data.

Ticket: http://tracker.phpbb.com/browse/PHPBB3-11525

PHPBB3-11525