Better subscription options

Do not merge without also merging https://github.com/alphagov/finder-api/pull/101

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.

The main addition to make this work is the ability to create a title
for the subscription from the chosen options, and the signup content
item. How the titles should be created is documented at
https://etherpad.mozilla.org/uK2bBcLupr

Also includes changes to support new format for storing choices in the content store.
@tommyp More like this line: https://github.com/alphagov/gds-api-adapters/blob/master/lib/gds_api/json_client.rb#L20 but the point I made is still valid: do you want to rely on gds-api-adapters to do the caching? If you swapped out gds-api-adapters for something that didn't have caching built-in, then suddenly this code would be making lots of requests.

For the cost of adding `@_content = `, I would add it. If nothing else, it avoids someone who isn't so familiar with gds-api-adapters wondering if there's a problem there.