This finally makes the licensing under AGPL-3.0-or-later explicit after
I got the okay from the kickstart source owners.
This also checks the REUSE compliance in a pre commit hook, and
therefore also in CI.
Works on #73
This contains changes on multiple files to associate
multiple employees with an order and make customers
able to add employees to their order.
Because of SQL JPA things, this will not be final.