Implement new styles for document filter summary

 - Large number prefixing summary
 - Remove grey boxes surrounding chosen options
Remove unused chosen class
Redesign feed and email buttons

 - Rename buttons and add text explaining them
 - Show panel with copy/pasteable link when clicking on "feed".
   This fixes the problem of people clicking on "feed" without
   knowing what it means, then getting a page full of scary XML.
Remove "all topics" and "all orgs" from filter
Display document type in document filter
Rename .document-filter-page to .documents-index
Remove duplicate documents-index layout CSS
Fix tests checking for filter result summaries

Looks like id was changed to class at some point and tests
weren't updated.
Move filter helper to documents-index view CSS

These styles are only used on the documents-index.
Remove unnecessary feed class
Add line to policies filter summary
Add comma to number of filter results
Add tests for JS filter pluralising
Merge pull request #1028 from alphagov/59495718-new-document-filter-summary-design

https://www.pivotaltracker.com/story/show/59495718

New document filter summary design