Schema editor uses wrong database

[Chainfire]

With a multiple database setup (and using "default": {} or with the dummy backend to catch programming errors), creating partitioned tables works, but add partitions to it does not.

For example, PostgresSchemaEditor::add_hash_partition uses a naked transaction.atomic() call. As the database to use (using=) is not specified, Django will create a transaction on the default database rather than the target database, and this will fail if no default database is configured. If a default database is configured, the transaction will execute there which is also not good.