Email subscription with filters

This pull request includes work to allow email signup with a list of tags in order to filter email subscriptions. This involves hard coding the list of facets on which drug device alerts can be filtered for email. The mechanism has however been built with other means of getting this facet information via an API rather than the schema.

Trello ticket here:
https://trello.com/c/9cA1JxhB/341-mhra-alerts-signing-up-to-either-drug-safety-alerts-or-medical-device-alerts-5-1

It also includes a piece of work to switch over subscription from using the govuk_delivery_api to using the new email-alert-api, as per this story:
https://trello.com/c/D7GNMvSv/348-swap-finder-frontend-and-specialist-publisher-to-use-new-email-alert-api-3

In order to handle some of the test in cucumber we have upgraded rspec and capybara.

This story is dependent of the deploy of http://www.github.com/alphagov/static in order to pull the new selectable styling for checkboxes found here:
https://github.com/alphagov/static/pull/468
I don't think this abstraction gets us anything here. If we swap out the API we use for getting artefacts we have to change way more than just the concretion in the ArtefactAPI, so given it's just a simple delegator to `content_api.artefact` I think we'd be better off here just calling the content_api.