[BUGFIX] Extended search with multiple custom fields support #1041

[mschwemer]

Extended Search in Backend Module with multiple values in Additional Fields (custom fields) cause empty search results. Bugfixes this by query each field on its own and limiting the search by the intersection of the custom field relations

See PR #1041. It was closed by deleting the target branch.

This PR serves as a reference for issue solution. Needs to be rebased for newer versions.