This commit add model classes closely matching the tables defined in
PowerDNS' schema as described at
https://doc.powerdns.com/md/authoritative/backend-generic-mypgsql/.
The commit includes the model definitions a schema migration including
PostgreSQL specific CHECK constraints and the registration in the Django
admin interface.
addresses #17
- 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