[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)


You shouldn't be using globals. ```$request``` already exists in this class and ```$db``` should be passed using the service container, too.