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.
I think it would be cleaner if the server set this in the context as a default like ``transaction_status_url: '/poll-wait-to-start'`` but maybe that's for another bug.