Invoices should list the ticket IDs

[malemburg]

This would help with badge pickups, since attendees often bring their invoices as proof.

Invoice currently just list the ticket code:

[matrixise]

Good point, for EP2021, we will some people with the invoice but not with the ticket number.

[malemburg]

Would be nice to have to EP2020 already, since we'll again use the ticket ID on the (virtual) registration desk.

[fpierfed]

The Invoice class is in assopy/models.py and already imports Ticket from conference/models.py

[ethancarlsson]

I'll get started on this one