Make Payment form credit / debit card changing available to backoffice

The front end currently allows switching payment forms (ie. from one processor to another - whether direct debit, or different credit card) whereas the backend form doesn't.

I have done some work on removing the pseudo-payment form from the contribution page & event page & putting it in it's own form. To make the js work we need to sync the field name so I am changing the front end field from payment_processor to payment_processor_id which is more accurate (although sadly requires more changes).

So far the code I have seems to be loading OK but
1) the js to flip the card type based on the card number isn't working back office because it's being assigned through the main form rather than the billing form. (this isn't a blocker).

2) I think we need to think about users on checksums - probably by passing it into the payment form. I'll check this

3) still some more QA needed

There are quite a lot of changes so I'll try to hit a balance between letting it go stale & merging too soon.