Implements fully centralized apps.

Implements fully centralized apps, by taking over the table creation.
It basically inserts the schema_name in front of the desired table references at:
* index creation
* table creation
* foreign key creation

Disables the table introspection when syncing tenant models, because otherwise
every model's table would appear as already created if they are already synced
into the public schema, because public schema is always in the search_path now.

Also eliminated "include_public_schema" as it is not needed anymore.
postgresql_backend.creation module takes care of every case.