Move the layout schema(s) to this repo

[olemartinorg]

The CDN repo has some schemas, and among them we have the layout.schema. While this file defines the contract used by both app-frontend-react and altinn-studio, changes to this file usually follows a certain route:

1. A new feature/fix is implemented in app-frontend-react, requiring changes to the layout schema
2. The change is made to a fork of altinn-cdn (because of missing permissions to create branches), and a PR will usually hang around for a bit too long before being merged (again because of missing permissions).
3. A long time after this is done, the options to control this from Altinn Studio might be implemented

As changes usually follows this route (and are rarely/never implemented in Studio first - that would just produce invalid layout files app-frontend cannot read), it makes sense to move this file to this repo. That would also keep updates to the file in sync with the latest release of app-frontend-react, as we'd push the changes to altinn-cdn at the time of release.