Adjust category filter to hide non-selected sibling subcategories
Adjust category filter to hide non-selected sibling subcategories

 - per conversation with Jeremy Y, this is still a bit experimental
 - would be even better if we could show just one category at a time
   for the subcats that appear in multiple categories.
 - also return selected category filter, this is so that we can access
   it from the template (see future commits).

https://trello.com/c/x6ANlaCf/339-filtering-based-on-category

alphagov/digitalmarketplace-buyer-frontend,alphagov/digitalmarketplace-buyer-frontend,alphagov/digitalmarketplace-buyer-frontend,alphagov/digitalmarketplace-buyer-frontend