Add ability to have defaults for user-data

[edwardhorsford]

We should add the ability for there to be defaults for user data. In this case, when the app starts values would be defined for data['foo'] rather than undefined.

Two potential use cases:

1: Testing 'return' or 'logged' in journeys - where we simulate a user returning to a previous application, it would be good to have their 'answers' pre-populated

2: Makes building 'admin' pages easier. I've done admin pages using auto-data store to easily change config for a prototype.

[NickColley]

I believe this is a duplicate of #272 which has more activity.

So will close this out.