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'm not sure I agree here. The main reason so many changes occurred when moving from the delivery api to the email alert api is the complete differences in the data required. In fact I believe the original delivery api url construction for example should have been pushed to this class instead of the Signup object itself as the api wrapper should simply care about titles and tags. This hides all complexity of how the api expects to receive tags (whether that be as a constructed url or a JSON hash) inside the wrapper class and isolates us from changes to the API. Id be interested to hear others thoughts on this though.