Started port to Django 2.1, Python 3, Docker

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.

gnuviechadmin/domains/views.py

@@ -4,16 +4,13 @@ This module defines views related to domains.
 """
 from __future__ import absolute_import, unicode_literals
 
+from braces.views import StaffuserRequiredMixin
+from django.contrib.auth.mixins import LoginRequiredMixin
 from django.shortcuts import redirect, get_object_or_404
 from django.views.generic.edit import CreateView
 from django.utils.translation import ugettext as _
 from django.contrib import messages
 
-from braces.views import (
-    LoginRequiredMixin,
-    StaffuserRequiredMixin,
-)
-
 from hostingpackages.models import CustomerHostingPackage
 
 from .forms import CreateHostingDomainForm