Add payment cancel (delete) to Payment REST API.
Add API for current payment and use it instead of the REST API.

This commit also includes:

* Remove order unload / reload code in Ember.
* Better error messages on payment error.
* Redirection to item list (voucher or donation) on payment error.

BB-655 #close
Merge pull request #445 from benkonrath/order-payment-fixes

Order payment fixes