Fix payment form resubmission before server response and other payments flow improvements

* Add client-generated timestamp to protect against duplicate form submissions when a user refreshes before any server response is issued (thus the PRG is never triggered)
* Add an alert warning to the payments dialog on submit to guard against page refreshes
* Keep track of failed payments that hit the API endpoint for hold authorization
* Scroll to bottom of header instead of top of payment container on the payment pages to prevent any alert messages from being hidden.