Implement overview of invoice accounts.

[SuperVK]

once this issue: GEWIS/sudosos-backend#148 has been merged, we can implement these endpoints to be used by the frontend.

Basic idea is that you can see all of the invoice accounts that still have money open, you can generate invoices for these accounts, and later mark them as paid. As well deleting invoices etc.

[JustSamuel]

Generating invoices and an invoice overview has been implemented, rebranding this issue to invoice account overview.

[JustSamuel]

I think this can be closed with the new invoice overview. Maybe we are missing something, but that would be out of scope for this issue.