Add --empty option to makemigrations
Remove redunant code from RunPython operation
Fix my slightly hasty autodetector changes
Add reference documentation for operations and stubs for schemaeditor.
Reverting unapplied migrations check away from being a system-level check.

This reverts commit 0ac13eccebb3d879f79b31585b1e81f655067179.
Fixed #21856: Don't crash runserver when DATABASES = {}
Removed test for migration check forgotten in 7e941ba67ceab8a74ba50509d13ed4298f8305cd.
Fixed #21951 -- Updated docs to use __str__ for Python 3

Thanks Tim Graham for the report and recommendations
Fixed #21064 -- Refactored common logic in ModelAdmin.add_view and change_view.