implement model changes

- 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

gnuviechadmin/domains/apps.py

@@ -0,0 +1,17 @@
+"""
+This module contains the :py:class:`django.apps.AppConfig` instance for the
+:py:mod:`domains` app.
+
+"""
+from __future__ import unicode_literals
+from django.apps import AppConfig
+from django.utils.translation import ugettext_lazy as _
+
+
+class DomainAppConfig(AppConfig):
+    """
+    AppConfig for the :py:mod:`domains` app.
+
+    """
+    name = 'domains'
+    verbose_name = _('Domains')