Better subscription options

The other half of https://github.com/alphagov/finder-frontend/pull/134 - Please don't merge without merging that PR.

Instead of relying on finder-api to ensure that all possible subscriber
lists are create for it, finder fronted is now responsible for creating
the list if necessary.

By no longer supplying a key in the email_signup_choices hash, and
making it an array, we can simplify a lot of our code.

This commit also renames ‘name’ to ‘radio_button_name’, to distinguish
it from the ‘topic_name’, which has been added. This is due to
sometimes wanting to display something different in the topic title of
a subscription from what was on the radio button.

Also include new option to add a prefix to a subscription’s subscription list title.