Save donor info

First draft of saving donor info.

1. Creates a new application to store pay.gov-related functions - this division should allow us to limit which urls are accessible simply by turning applications on and off
2. Move payxml and several related functions from the view code into the new app
3. Save the generated xml when a user hits the confirmation page
4. Remove some unneeded fields from the review submission form
My suspicion is that we'll need more time than this for the default expiry. Should this become a setting so that we can change it easily?