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.views.CreateHostingDomain
- define new URL create_hosting_domain in domains.urls
- add domains app URLs to gnuviechadmin.urls
- add templates domains/base.html and domains/hostingdomain_create.html
- link from hostingpackage detail page to domain creation view