[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.

PHPBB3-11525
[ticket/11525] Fix doc blocks

PHPBB3-11525
[ticket/11525] Prefix id parameter with 'g' again when its a group avatar

PHPBB3-11525
Merge pull request #2 from nickvergessen/marc/ticket/11525

Marc/ticket/11525
[ticket/11525] Compare correct array to expected value

PHPBB3-11525
[ticket/11525] Fix expected value of group avatar test

PHPBB3-11525
Merge pull request #3 from nickvergessen/marc/ticket/11525

Marc/ticket/11525 Part 2
[ticket/11525] Use foreach instead of array_walk in method clean_row()

This approach is cleaner and probably even faster the previous ways that
included using array_walk() or array_map() and other helper functions
and methods.

PHPBB3-11525
Merge pull request #1815 from marc1706/ticket/11525

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