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.
Is ``app.transaction[jwt]`` set by the session? Because in the case of finishing a payment, the JWT might still be in the session. In other words, maybe a truthy ``app.startView`` should override any kind of transaction session lookup.