Feature spec the email subscription mechanism

Before starting work on changing our subscription mechanism, lets make sure its specced. The test case here uses drug-device-alerts. This means we have some duplicated helpers from our CMA Case features, we want to refactor this out in the future so we no longer rely on the finder for email subscription pages
Refactor out the use of Finder in email controller

We should not require the use of a Finder when creating email alert subscriptions so instead lets wrap up the content_api for reuse elsewhere and pull only the email signup artefact. We can then use this to simply generate the title and unique identifiers for email topics within govuk_delivery
Allow Facet classes to be used by signup views

Passing down the facet schema hash allows us to build a FacetCollection for use in the view. We can then add these parameters to our unique url passed to the delivery API in order to generate unique urls for each individual topic.
Push signup url generation int EmailSignupPage
Make parameter selection use facet keys

In order to be inclusive rather than exclusive about the parameters we expect, make use of the keys we allow from emailable facets rather than excluding default rails parameters
Upgrade gds-api-adapters
Swap out delivery_api for email_alert_api
Style checkboxes on the email signup pages