Fixed #16759 -- Remove use of __deepcopy__ in qs.clone()

The original problem was that queryset cloning was really expensive
when filtering with F() clauses. The __deepcopy__ went too deep copying
_meta attributes of the models used. To fix this the use of
__deepcopy__ in qs cloning was removed.

This commit results in some speed improvements across the djangobench
benchmark suite. Most query_* tests are 20-30% faster, save() is 50%
faster and finally complex filtering situations can see 2x to order
of magnitude improvments.

Thanks to Suor, Alex and lrekucki for valuable feedback.
Updated deprecated test assertions
Fixed #17541 -- Fixed non-saved/nullable fk querying
Fixed #19576 -- Use `six.with_metaclass` uniformously accross code base.
Fixed #19555 - Removed '2012' from tutorial 1.

Thanks rodrigorosa.lg and others for the report.
Added sphinx substitutions in place of hardcoded version numbers.

Refs #19571
Fixed #19546 - ensure that deprecation warnings are shown during tests

refs #18985
Remove inaccurate comment regarding language names.
Skipped deprecation warning test on Python 2.6

Refs #19546. On Python 2.6, DeprecationWarnings are visible by
default.
Fixed lockups in jenkins, refs #19546.
Fixed #19586 - Removed URL_VALIDATOR_USER_AGENT from setting docs.

It was removed in Django 1.5, not deprecated.
Addeded CSS to bold deprecation notices.

Thanks Sam Lai for mentioning this on the mailing list.
Fixed #19581 -- ensure unique html ids with CheckboxSelectMultiple widgets

ID check is now done the same way as MultipleHiddenInput.
Fixed #19573 -- Allow override of username field label in AuthenticationForm
Fixed #19588 - Added create_superuser to UserManager docs.

Thanks minddust for the report.
Fixed #10239 - Added docs for modelform_factory

Thanks ingenieroariel for the suggestion and slurms for the review.
Fixed #19437 - Clarified pip install instructions in contributing tutorial.