Revoking unused vouchers

[A-Guldborg]

As a board member, I should be able to revoke an unused voucher (soft-delete).

Subtask:

• Ensure using a revoked voucher is not possible (perhaps change error message to "Invalid voucher. Has it been revoked?")
• (Possibly) As a manager, I should be able to revoke my own issued unused vouchers (soft-delete)

Semi depends on #312 - It is possible to create revoke endpoint without the query endpoint