Fixed #17051 -- Removed some 'invalid' field error messages

When the 'invalid' error message is set at field level, it masks
the error message raised by the validator, if any.
Deprecated django.views.defaults.shortcut.
Fixed #17502 -- Made joining in inheritance cases consistent

The original problem was that when filtering two levels up in
inheritance chain, Django optimized the join generation so that the
middle model was skipped. But then Django generated joins from top
to middle to bottom for SELECT clause, and thus there was one extra
join (top->middle->bottom + top -> bottom).

This case is fixed in master as the filtering optimization is gone.
This has the side effect that in some cases there is still extra join
if the SELECT clause doesn't contain anything from middle or bottom.
Corrected typos in the 1.6 release notes
Discouraged filing PRs that won't be noticed.
Merge pull request #901 from giamfreeg/master

Fixed an erroneous import in example code of docs (class-based-views/intro).
Fixed #20053 -- Fix `index_together` documentation
Merge pull request #905 from cyberj/ticket_20053

Fixed #20053 -- Fix `index_together` documentation
Fix #20054: Removed links to modwsgi.org.
Fixed #16110 -- Fixed GeometryField odd behaviour regarding null values

Thanks slinkp for the report and the initial patch.
Fixed #20055 -- Add url, email, and number input types to admin CSS.
Fix 20061: remove out of date comment.
Fix #20058: Make compilemessages use stdout instead of stderr.
Merge pull request #909 from bmispelon/ticket-20061

Fix 20061: remove out of date comment.
Merge pull request #906 from bmispelon/ticket-20054

Fix #20054: Removed links to modwsgi.org.
Merge pull request #910 from bmispelon/ticket-20058

Fix #20058: Make compilemessages use stdout instead of stderr.
Fixed memory leak in tests

The AssertNumQueriesContext didn't reset connection's use_debug_cursor
in case there was an exception. This resulted in leaking query strings
into connection.queries. Maximum memory use pre-patch was around 700MB,
post-patch it is around 200MB for Django's test suite.
Removed debug code
Updated bundled version of six.
Stopped using non-standard __globals__ and __code__ attributes.

Some alternative implementations don't have them.

Closes #19944.