SPA success/error/failure views (bug 1034165)

UPDATED (7th July)

* Move to using same page in the SPA for success in Bango + Reference provider (as well as Boku).
* Redirect from SPA provider error views to a 'payment_failure' view in SPA which will display the correct error message based on the error messages lookup.
* Add the  view to serve the SPA for error pages called by the provider and the payment failure page.

Note: All the provider views and tests are copies of the pre-existing views + tests with modifications to suit this usage.
These would all be cleaner with an include:

    (r'^mozpay/spa/', include('webpay.spa.urls')), ...

The main reason is that we probably won't want to prefix them with ``spa`` forever.