Add search facets to the results

By adding the facets as data attributes, we have the hooks we need to
provide client side filtering without making calls to the server.
Create and serve application.js

…with placeholder code to confirm it is working.
Filter based upon support checkboxes

These are inclusive filters (OR), whereas the dropdowns are exclusive
(AND) so need to be treated differently.
Update the count of matches after filtering
Filter based upon select dropdowns

These are exclusive filters (AND). 

Updates the HTML template to have the select name attribute and the 
data-* attribute on the results match, to keep the code simple.
Hide submit button once filtering in place
Tidy up
Remove duplicated styles

These are available from the govuk_frontend_toolkit gem which is 
already included.
Update gds-api-adapters
Refactor how Imminence and Content API data is merged.

Simplifies the merging of facet values into scheme data.
Adds a helper to format facet values in the view.
Add poltergeist
Test client-side filtering
Fix copy/paste error
Update browser history after changing filters

This gives us proper URLs that can be shared even when JS is being used
to modify the page.

This means that client-side filtering now only applies to browsers that
support the History API.
Merge pull request #42 from alphagov/clientside-filtering

Client side filtering of results