Display only payment method selected partial (backend)

Drop a bunch of repeated ids on html

Conflicts:
	backend/app/assets/javascripts/admin/payments/new.js
	backend/app/views/spree/admin/payments/source_forms/_gateway.html.erb
	backend/spec/features/admin/orders/payments_spec.rb
Display cards as per gateway on backend new payments

Otherwise users could try a transaction against a gateway which won't
understand the cc profile id
Allow creating a new card on any payment method
Fix validation translation
Admin::Orders test completed? instead of complete?

Checkout steps are customizable, 'complete' may not be the only state in
which an order is considered complete.

Fixes #4751