Add wait-to-finish for SPA (bug 1024451)

This branch adds a wait-to-finish view that serves the SPA. This is so we can catch the callback from the provider post-payment and do the final stages before completion.

The tests are mostly a duplicate of the pre-existing provider wait-to-finish ones. This helps to catch any issues when removing code after we've switched over to the SPA.
So the user gets redirected to this page? Why the full reload instead of returning the URL in the API?