[ticket/12890] Search by custom profile fields for members

Still a work in progress, requires work for field-specific scenarios such as the date field and requires work on finishing the templates for memberlist

[PHPBB3-12890](https://tracker.phpbb.com/browse/PHPBB3-12890)


Please check if row count is even or odd instead: https://wiki.phpbb.com/Tutorial.Template_syntax#Conditionals.2FControl_structures