Moved sys.path-extending decorator to django.test.utils and used throughout test suite.

Thanks Aymeric for the suggestion.
Moved RequestSite and get_current_site.

Following the app-loading refactor, these objects must live outside of
django.contrib.sites.models because they must be available without
importing the django.contrib.sites.models module when
django.contrib.sites isn't installed.

Refs #21680. Thanks Carl and Loic for reporting this issue.
Fixed typo. Refs #21836.
Fixed #21702 -- get_model('app_label.ModelName').

Also added tests for get_model.
Took advantage of the new get_model API. Refs #21702.
Fixed #21877 -- Renamed django.apps.base to config.
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.