GDPR Compliance: make it possible for a subscriber to cancel sign up

[cyberflaneuse]

In the same context of #3011, making it possible for users who subscribed to a demo or a service on a mutlti-tenant GlobaLeaks setup to delete their platform. E.g. public administration enters employee data for a demo platform (which is currently being done manually in the project WBPA), should be able to delete it.