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.
Isn't that what https://github.com/alphagov/gds-api-adapters/blob/master/lib/gds_api/base.rb#L14 does?