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 new domains.apps.AppConfig to allow translatable app description for
domains app
- link domains to a customer
- extract common functionality from domains.models.MailDomain into abstract
domains.models.DomainBase
- add separate domains.models.HostingDomain to allow for generic external
domains
- add new hostingpackages.models.CustomerHostingPackageDomain to assign hosting
domains to hosting packages
