Remove unused preference accessors
Fix Skrill Quick Checkout billing integration

- remove invalid preferences (server and test mode)
- remove arbitrary distinction of "global options"
- use order's currency instead of setting it via preference
- reverse_merge all options so they can be overridden
Massive improvements for handling Skrill transactions

Controller/View layer:
- extract functionality to separate controller rather than decorating the CheckoutController
- iframe loads from the app itself (simplified hiding of parameters)
- use payment identifier instead of order ID as transaction ID

Model layer
- move status validation logic from controller to model
- remove magic numbers for payment status
- validate signature to verify payment hasn't been tampered with
- save all received params to database (facilitates debugging)