Decide the home redirect per request instead of querying at import time,
replace the removed _get_placeholder helper, stop shadowing the
is_superuser field, and set the seeded home page as the CMS home.
Each site gets a home page and four child pages, and there are twelve
dummy customers. The dummy admin password is now fixed through
DUMMY_ADMIN_PASSWORD. --purge removes all of it.
The seed_dummy_data command creates one CMS page per site and a dummy
admin and customer, idempotently. --purge removes exactly what it
created. It runs on start when SEED_DUMMY_DATA=True and a failure does
not stop the web server.
