Add support for a function getTotalAmount that could be used to retrieve amount from form if defined

If Stripe is not using the same currency as the payment was made we need to convert the fees/net amounts back to the CiviCRM currency

The "name" parameter on a set of checkboxes where at least one must be checked must be the same or validation will require all of them!

Add jquery form event 'crmBillingFormNotValid' so 3rd-party integrations can re-enable custom submit buttons etc.