Switch to new choices content format

As changed in 77e65b0 on finder-api
Finder fronted find or create subscriber list

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
Merge pull request #134 from alphagov/better-subscription-options

Better subscription options