Deprecates the "--plain" option.

This includes the ability to collapse inlines by specifying a class named
'collapse'.

Added unapplied migration count and the list of unmigrated apps.

For all practical purposes, there are no common cases for which a
website cannot be deployed with HTTPS.

This fixes a regression introduced by a5f6cbce.
Thanks Gavin Wahl for the report and Tim Graham for the review.

Thanks to Trac alias benzid-wael for the report.

This fixes a regression introduced by 87144036.

Thanks to Geoffrey Sechter and the Django NYC group for the report and Markus
for the review.

There is no need to wrap the save() call in transaction.atomic() as
it's already done down the call stack in Model.save_base().

Thanks to Trac aliases gerricom for the report, raphaelmerx for the
attempts to reproduce and Sergey Fedoseev and Tim for the review.

Refs #16891

Thanks Ben Kraft for the report, and Simon Charette for the review.

Thanks to bmispelon and uruz for the initial patch.