Django manage steps in compose startup sequence

[mapto]

• Renamed environmental variable DJANGO_SUPERUSER to DJANGO_SUPERUSER_USERNAME and added DJANGO_SUPERUSER_EMAIL. This allows us to directly call python manage.py createsuperuser --noinput.
• Updated docker-compose.yml:
  • added healthchecks, depends_on condition and restart.
  • expanded compose command to run django management steps on startup
• DJANGO_ALLOWED_HOSTS now actually works with more than one host (space separated)

[stefanwayon]

TBH I don't remember the details of this, but your changes definitely include the ones in the other PR so happy to close that one. :-)