Force inclusion of public schema during schema migration.

Looks like these were originally written before there was a separation
of the public and per-tenant schemas; I don't see any issues with
including the public schema, since doing a migrate there doesn't
actually create any per-tenant objects for the tenant with
schema_name='public'.

For the case where a model exists in *both* public and tenant schemas,
we force the issue with include_public=False.