Enable admin to change event payments to the status "resolved"

[mariobehling]

All invoices of the system are listed on https://eventyay.com/admin/sales/invoices. Currently there is no option of an admin to mark a payment as "resolved". This is necessary if the system owner wants to allow organizers to use the system free of charge. For now the easiest way to achieve this is by having status options in a Status column as a dropdown.

1. Therefore please implement:

• a Status column where the admin can change a payment to the following states:
  • Due
  • Paid
  • Refunding
  • Refunded
  • Resolved
• When the user has the status resolved the system should regard this similarly to the paid status, but it should not add the cost of the invoice as income in the system. Basically, the user did not pay it, but the user also does not have to pay it anymore.

2. Also add the status "Resolved" for the specific user in their invoice overview at https://eventyay.com/account/billing/invoices/all