Imported custom user classes in tests depending on it

Without those imports, affected test files cannot be run
independently. Refs #21164.
Amend my description in committers.txt.
Fixed #15888 -- Made tablename argument of createcachetable optional

Thanks Aymeric Augustin for the report and the documentation and
Tim Graham for the review.
Fixed #21027 -- Updated tutorial 5 docs to link to management shell command page.
Fixed #21264 -- Incorrect RST usage in docs
Fixed #18866 -- added validation error for verbose_name longer than 39 characters

Added a validation error check when creating the permissions for model, to avoid
cryptic database error when the verbose_name is longer than 39 characters
thanks elena for reporting it
Merge pull request #1733 from joaoxsouls/#18866

Fixed #18866 -- added validation error for verbose_name longer than 39 characters
Fixed #21210 -- Documented when runserver doesn't auto-restart.

Thanks gergely at polonkai.eu for the suggestion.
Clarfied the ModelForm docs with respect to generated vs. declared fields.

The ModelForm docs suggested that fields defined declaratively override
default fields generated from the form Meta.

This is conceptually wrong, especially with inheritance in mind. Meta is
usually defined on the topmost ModelForm subclass, while fields can come
from anywhere in the MRO, especially base classes; therefore we suggested
that something defined in a base class override something from a subclass.

This patch rephrases the docs around the idea that Meta is used to generate
*missing* fields.

Refs #8620, #19617.

Thanks @mjtamlyn and @timgraham for the review.
Fixed #20962 -- Documented that template tag render() may raise exceptions.

Thanks Keryn Knight for the report.
Fixed #20910 -- Added a "snippet" sphinx directive to allow prefixing a filename.

Thanks Marc Tamlyn for the suggestion.
Added flake8 info to setup.cfg.

This config allows a clean run of flake8. Some of the ignored
codes will be remove over time as we clean things up.
Close file after tests added in 945e033a69.

Avoids failures on Windows. Refs #8918.
Fixed #21269 -- Don't crash when CommandError contains non-ascii

Thanks kontakt@eikefoken.de for the report.