Add hidden_facet_tag
Add hidden_facet_tag

Finder frontend relies on the value of
hidden_facet_tag to choose what to render as
hidden text for screen readers. The absence of this
attribute was causing bug https://trello.com/c/
iyd3UY3z/734-fix-hidden-text-for-screen-readers-on-finders

alphagov/rummager,alphagov/rummager