display bugs of sepa payments status in the order view

[chr-chr]

Steps to reproduce:

• have an order with a sepa debit
• export it
• "This order has been paid via SEPA debit. It was part of the export file exported ... link to export file"
• refund an amount
• order gets status 'pending'
• sepa debit shows up "This order has been planned to be paid via SEPA debit but " ...

However, it was exported.

I'll expect the debit status 'was exported' doesn't and can't change can't be undone ...

[chr-chr]

My attempt to fix the above: 9512867

However, then I noticed another issue:

• have an order with two or more sepa debit payments (yes, possible!)
• the list of sepa payments: status info line and link to "export file" are wrong:

98d7326

order.sepaexportorder_set => payment.sepaexportorder_set