Refactor Finder to be a Presenter

As we move away from using Finder API, we want to store all the
information for a Finder inlcuding the schema in the Content Store.

This commit is the start of the work that will allow this to happen.
By moving the presentation logic of a Finder from the model to a
Presenter, we follow a more standard Rails pattern. This also allows to
get rid of the horrible `Finder#get` method which fetches the schema
from Finder API.
Refactor facets to use ContentStore response

Now that Finder Schemas are inside the details hash in the response
returned from the ContentStore, we can refactor the facets used and the
Parser.

This also means we can remove the OpenStruct option generation from the
parsers, since all the options are OpenStructs.

While doing this work, I discovered a lot of leftover code that isn't
used anymore such as the `SelectFacet#include_blank` and the methods
related to building native Select boxes so I removed it also.

This commit also contains the updates to the tests.
Remove FacetCollectionParser and Spec

As this is now an OpenStruct, we don't need a seperate class to return
us Facet objects. This logic now happens in the Presenter and talks
directly to the FacetCollection.
Remove RadioFacet and Spec

After Finders were developed we found out through User Research that
Radio Facets were confusing for searching, so we phased them out and
used smaller select facets.

This commit removes the radio facet and the spec.
Remove Finder API from FinderFrontend

This commit removes the call to the Finder API for the schema and
removes the schema beind passed around in Finder Frontend, as it's no
longer used.
Replace Finder API in tests

This commit removes the stubbed response from Finder API and instead
adds the schema to the stubbed ContentStore response.
Use FinderPresenter in Email Subs Controller

As we now use a Presenter, we can call this instead in the email alert
subscriptions controller. This commit also updates the test and includes
webmock to stub out the API in RSpec.
Merge pull request #125 from alphagov/finder-as-presenter

Finder as presenter