Improve the way macros are imported from GOV.UK Frontend

At the minute all of the the component macros are included as part of the default layout:

https://github.com/alphagov/govuk-prototype-kit/blob/3b6870fc6b09689a54793aefc76485200c5db324/app/views/layout.html#L6-L30

We do this so that users do not need to import the components themselves – despite the examples in the Design System always including the import, we tell them not to include it.

This means that:

• when we add a new component to GOV.UK Frontend, we have to remember to add it to the list of imports
• components are only imported into the default layout, and not the unbranded layout
• GOV.UK Frontend works differently to any other ‘extensions’ the user has installed

We should explore ways to auto-load the components in all of the provided layouts, possibly via the extensions framework.

[edwardhorsford]

A side advantage of a separate include is that that file could live outside of app so more easily be updated. Keeping layout for user-side changes.

[joelanman]

components are now all imported automatically via the GOV.UK Frontend Prototype Kit config file