Describe the change

After #2086, data retention periods are defined in the services.py for each app. For clarity, it might be good to centralize this information.

Motivation

Keep a clear overview of personal data we are keeping and deleting

Current implementation

Every app should have a services.py execute_data_minimisation service that does this.

Suggested implementation

Some ideas:

• Put in settings.py a dict with for every model, a time delta object when data should be removed
• Put this information in every model
• Create our own MemberForeignKeyField that subclasses regular ForeignKeyFields, but additionally requires retention_period as an argument. This way, we might even be able to implement the data minimisation centrally

Additional context