ADD: Labels for search filters should be translatable per search field

FIX: changes suggested by @Daxter

REFACTOR: proper file with method translation_per_search_field(field)
Merge branch '2160-CLEAN-labels-for-search-filters-should-be-translatable-per-search-field' of github.com:goodylabs/active_admin into 2160-CLEAN-labels-for-search-filters-should-be-translatable-per-search-field