[feature/avatars] Pass phpbb_user object to get_custom_html()

Pass the phpbb_user object to function get_custom_html(). This object is
used in that method. Also fixed incorrect arguments to get_custom_html()
in phpbb_get_avatar().

PHPBB3-10018
[feature/avatars] Pass phpbb_user object to prepare_form_acp()

The phpbb_user object might be used for language variables. Pass it as a
function argument to prepare_form_acp() instead of using globals.

PHPBB3-10018