This is the result of Christopher Medrela's 2013 Summer of Code project.
Thanks also to Preston Holmes, Tim Graham, Anssi Kääriäinen, Florian
Apolloner, and Alex Gaynor for review notes along the way.
Also: Fixes#8579, fixes#3055, fixes#19844.
Added comments in the three empty models.py files that are still needed.
Adjusted the test runner to add applications corresponding to test
labels to INSTALLED_APPS even when they don't have a models module.