Improved search filters, legacy edit fix, other fixes

• Remove faulty 'not available' label from search results - #593
• Fix link to edit unpublished (no links) datasets - #588
• Add support rake tasks for deleting datasets - #571
• Fix dismissing blue "we've been improving" message - #590
• Fix accessibility issue for sort order button - #586
• Change search filters to only show applicable options - #587
• Fix CSV preview overflowing page bounds - #592