And accessorily added missing bits fixing #23293.
Forward port of 2cbafd81 from stable/1.7.x

Thanks jonash for the suggestion and initial patch.

Windows doesn't like that and it causes 114 failures in the
admin_scripts tests with Python 2.x.

Thanks buettgenbach at datacollect.com for the report and patch.

Thanks MarkusH for the report and Florian for review.

Thanks SpaceFox for the report.

Thanks homm for the report, Cea Stapleton for patch improvements
and Ian Cordasco, Christian Schmitt and Tim Graham for the review.

mimetype was deprecated in 1.5 per 11ec0253.

Refactored openCalendar function from DateTimeShortcuts.js. Now, when
entered manually in the input field, the date will show up correctly on
the calendar for locales that don't use "-" for separator.

Thanks charettes for revivew and Alexey Boriskin for some of the patch.

Thanks Baptiste for the suggestion.

This limits large variables and avoids non-utf-8 in the TECHNICAL_500 output.

Thanks jonash for the initial patch; refs #20133.

Thanks giulettamasina for the report and Tim Graham for the review.

Also never create deferred model when no attrs are deferred.