When STATIC_ROOT wasn't set, collectstatic --clear would delete
every files within the current directory and its descendants.

This patch makes the following changes:

Prevent collectstatic from running if STATIC_ROOT isn't set.

Fixed an issue that prevented collectstatic from displaying the
destination directory.

Changed the warning header to notify when the command is run
in dry-run mode.

Thanks Aymeric Augustin for the report and the review.

Thanks Jannis and Marc for the feedback.

Fixed #21717.

Fixed #21679.

Fixed #21683.

This check will miss duplicates until the check for duplicate labels is
added.

Refs #21679.

It doesn't account for app configs.

Refs #21679.

Thanks Marco Badan for the report.

Thanks ken.nelson at maclaren.com.

Fixed #21715.

Thanks Anssi for noticing this mistake in a refactoring.

Thanks cjerdonek for the suggestion.

This avoids duplicating code.

To resolve the conflict between application names.

Now that the refactorings are complete, it isn't particularly useful any
more, nor very well named. Let's keep the API as simple as possible.

Fixed #21689.

Many thanks to @bitdancer and @aaugustin for answering my stupid questions about (mail)encodings and pointing me in the right direction.

Thanks ashley at ashleymills.com for the suggestion.

Changed console and filebackend to use msg.as_bytes to output the data as it would get send via smtp.

Thanks Keryn Knight for the report.

After the recent series of refactorings, there's no reason to keep
two distinct methods.

Refs #21681.

To the best of my understanding, since populate_models() is now called
as soon as Django starts, it cannot be called while a models module is
being imported, and that removes the need for postponing.

(If hell breaks loose we'll revert this commit.)

Refs #21681.

Since it triggers imports, it shouldn't be done lightly.

This commit adds a public API for doing it explicitly, django.setup(),
and does it automatically when using manage.py and wsgi.py.

Refs #1796, #21676.

Currently it's 3.3.

Various small fixes while I was proof-reading.