[ticket/12426] User search to allow search/filter by custom fields
Memberlist search/filter to add custom fields

Rework of the memberlist.php searc so the users can bi filtered by
all custom fields visible in memberlist.

PHPBB3-12426
[ticket/12426] User search to allow search/filter by custom fields
Memberlist search/filter to add custom fields

Add Template variables to memberlist_search.html

PHPBB3-12426