Use attr to define start view (bug 1036935)

This allows webpay to define a starting view in the SPA.

This means we can modify the existing provider views to just serve the SPA along with a data attr to tell Spartacus what view should be used.
can this just throw an exception? Or maybe render an error page? It seems like making the function a no-op would be confusing. Wouldn't the user see a blank screen?