Integrate with accounting app to emit invoices

[GabrielMajeri]

Due to legal requirements, we will have to issue invoices for the users who choose to pay with a didactic career premium card. For these invoices to be valid, they also have to be saved in the University's financial accounting database, which is handled by the EMSYS application.

I will initiate a discussion with them to see whether they can provide us with an API to generate and export invoices. We will then also have to send a copy by e-mail to the payer.