Fixed #19596 -- Use `_default_manager` instead of `objects` in the auth app.

This is needed to support custom user models which don't define a manager
named `objects`.
Fixed #19523 -- Improved performance of Django's bash completion

Previous version took about 150ms to source, even on a warm cache,
primarily because it forks+execs /usr/bin/basename 44 times.
This patch makes it faster by a factor of 5 (and I imagine that a little
more thought would reduce the time to effectively zero).
Fixed #19640 - Added inlineformset_factory to reference docs.

Thanks wim@ for the suggestion.
Merge branch 'master' into 19552