Advanced search as a separate page. #refs #582

- Removed drop-down version of advanced search.
- Disabled simple search form when on advanced search page.
- Cleaned up some of the advanced search stuff.
- Changed "More Options" button to an "Advanced search" link.
Extraced controller method to separate helper.

- Extracted CatalogController#advanced_search_fields helper method
  to a separate helper method in AdvancedSearchHelper