Use django-tenants clone tenant for faster tenant creation

[atodorov]

Rationale: the current create_tenant() function triggers migrations and sometimes they take longer to apply which results in web timeouts. So far that didn't result in tenants with broken schema but it is possible and is also frustrating to users.

django-tenants supports the clone_tenant command, https://django-tenants.readthedocs.io/en/latest/use.html?highlight=clone#clone-tenant and IIRC in the past that wasn't working well but now it is. We've got to test it and make sure it is well covered both upstream and inside our own test suite (and also measure speed of execution) but it sounds like this is what we need to be using.