"Pluggable" payment platform registration, Renamed Payee.user to Payee.userservice to avoid confusion, Ajax claiming now properly updates the "View" href on success.

 - [apwan.core.payment] has been split up and put into a package and now uses a registry which gives us a more "pluggable" payment platform system
 - Everything (hopefully) has been updated to use the new payment platform registry system.
 - Payee.user field has been renamed to Payee.userservice to avoid confusion with users / user profiles.
 - [claim.js] was still using recipient id's, so has been updated to use slugs.
Removed some unneeded imports and refactored PaymentPlatform.build_info_dict to give more descriptive variable names.