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.
- implement domains.forms.relative_domain_validator
- use the validator for domain field validation in
domains.forms.CreateHostingDomainForm
- use the validator for subdomain field validation in
websites.forms.AddWebsiteForm