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 had a discussion with @bestie at the time around the testing method here. We felt that the helpers used in gds-api-adapters were hard to maintain and hard work with. We expect gds-api-adapters to be relatively well tested in isolation and so stubbing at the http layer is complex and unreadabkle to some degree. We felt that simply stubbing the gds-api-adapter directly was much easier to understand and involved less cognitive load for the developer. If the team feels that we should test at the http layer then this is something we should discuss.

This also follows the same pattern as some of the other api helpers used in specialist-publisher here:
https://github.com/alphagov/specialist-publisher/blob/master/features/support/delivery_api_helpers.rb