Fixed #8033 -- Explained app registry error during translation setup

Thanks Tim Graham and Aymeric Augustin for the review.
Cleaned temp dir in symlinks_supported()

Refs #21482.
Ensured cache tests do not leak temp dirs

Refs #17215.
Created import-time test temp dirs in known location

Refs #17215. In the same spirit as 5de31cb8cb.
Fixed #22888 -- Removed some unnecessary text in tutorial 3.
Fixed test failures introduced by refs #22881.
Fixed problem introduced with #refs 13711.
Removed 'to_field' from ForeignKey(settings.AUTH_USER_MODEL) in admin's initial migration.

This enables using a custom user model with a primary key with a name
different than 'id'; refs #22889.
Refactored common code in handlers/base.py and urlresolvers.py
Fixed typo: MultipleModelChoiceField -> ModelMultipleChoiceField.
Simplified test of contrib.auth.tests.
Fixed #17903 -- Modified ModelBackend to eliminate permissions on inactive users.

Thanks to @SmileyChris for the report and @timgraham for review.
Fixed #22487: Don't flush out data from before normal TestCases
Fixed #22889: Unneeded to_field in fk.deconstruct causing swap issues
Fixed #22708: Typo in autodetector base dependency gen
Fixed #22880 -- Added FAQ entry about UnicodeDecodeError

Thanks Víðir Valberg Guðmundsson for the report and
Tim Graham for the review.
Restored is_anonymous() check in ModelBackend permission checking removed in refs #17903.

Thanks Florian Apolloner for raising the issue.
Fixed #21668 -- Return detailed error page when SuspiciousOperation is raised and DEBUG=True

Thanks GDorn and gox21 for report.

Thanks Tim Graham for idea and review.
Fixed #22796 -- Added a more basic explanations of forms.

Thanks bmispelon, kezabelle, jorgecarleitao, and timgraham for reviews.
Fixed typo in middleware docs.

Function definition should have an ending colon.