'CASCADE' is an undocumented no-op in MySQL, so to avoid confusion Django no longer outputs it.

This is a security fix.

This is a security fix.

Thanks Tim Graham for the initial patch.

This allows bringing back the behavior of Django < 1.7.

Also fixed the check for the app registry being ready in
AppConfig.get_model(s), which was inconsistent with the equivalent check in
Apps.get_model(s). That part is a backwards-incompatible change.

Since AppConfig now has a reference to its parent Apps registry,
it can look up the models there instead of receiving them in argument.

It was inconsistent with the equivalent check in Apps.get_model(s)
because I made incorrect assumptions when I wrote that code and
needlessly complicated readiness checks.

This is a backwards-incompatible change.

Thanks Anssi Kääriäinen for the initial patch and Anssi, Simon Charette,
and Josh Smeaton for review.

This allows running tests w/o installing Django itself by only pointing
the Python path.