Fixed #21880 -- Added missing items to django.utils.timezone.__all__.

Thanks Wim for the report.
Fixed #21866 -- Replaced "COMMIT" in managmement command SQL with backend hook.
Fixed some missing/extraneous new line warnings.
Fixed #19774 -- Deprecated the contenttypes.generic module.

It contained models, forms and admin objects causing undesirable
import side effects. Refs #16368.

Thanks to Ramiro, Carl and Loïc for the review.
Minor cleanup.
Removed superfluous uses of TransRealMixin.

The translation.override context manager cleans up after itself. As a
consequence this mixin isn't needed any more in many cases.
Removed TransRealMixin.