Start-to-finish interaction complete

This finishes the first version of the mock pay.gov server by adding a final redirect page.

* Reworked configs to better match pay.gov docs
* Split out `access_settings.py` from the `views.py` code; allowed settings to be overridden a few different ways
* At the credit card "entry" page, user can cancel or continue
* When the user loads the redirect page, the agency server is contacted with the results
* The user then clicks "continue" or "return" or what have you and hops over to the error/success page of the agency
* Lots of testing tweaks