Update NHS App prototype kit to use published package

[frankieroberto]

Now that nhsuk-prototype-kit exists as a public npm package, the NHS App prototype kit can be updated to share the same code, but with any extra bits needed for the app.

Some options:

1. https://github.com/nhsuk/nhsapp-prototype could become a template repo
2. https://github.com/nhsuk/nhsapp-prototype could be retired and replaced with guidance somewhere on how to add NHS App frontend and any templates to the standard NHS prototype kit

Probably the first one is better?