This commit is a rough port to Django 2.1, Python 3 and a Docker based local
development setup. Tests fail/error but migrations and the web frontend are
already runnable. Task queue functionality is untested and translations seem to
have trouble.
- add managemails.apps to give a more meaningful title in the admin site
- add verbose names to MailAddressMailbox fields
- implement managemails.models.Mailbox.get_mailaddresses and a corresponding
property mailaddresses
