Fixed #19617 -- Refactored Form metaclasses to support more inheritance scenarios.

Thanks apollo13, funkybob and mjtamlyn for the reviews.
Fixed #8620 -- Updated the Form metaclass to support excluding fields by shadowing them.
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.
Fixed #21266 -- Fixed E201,E202 pep8 warnings.
Fixed #21223 -- Added form.id_for_label to topic guide.

Thanks ebertti at yahoo.com.br for the suggestion.
Fixed a webdesign template tag docstring to prevent parsing as metadata.

Previously admindocs would throw an error when processing it:
"Error in "default-role" directive: no content permitted."

refs #6681
Handle Apps with South migrations as unmigrated apps.
Merge pull request #1382 from loic/ticket19617

Fixed #19617 -- Refactored form metaclasses to support more inheritance scenarios.
Fixed #21006 -- Added example code for overriding methods on an InlineFormSet.

Thanks treysta at gmail.com for the suggestion.
Made further improvements to the fix for ticket 21006
further improvement for ticket 21006