Fixed #19637 -- Ensured AdminEmailHandler fails silently

Thanks lsaffre for the report. Refs #19325.
Fixed #18051 -- Allowed admin fieldsets to contain lists

Thanks Ricardo di Virgilio for the report, Mateus Gondim for the
patch and Nick Sandford for the review.
Fixed error introduced when testing patch for 013db6ba85

Shame on me.
Simplified a i18n test.
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.