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 #7261 -- support for __html__ for library interoperability

The idea is that if an object implements __html__ which returns a string this is
used as HTML representation (eg: on escaping). If the object is a str or unicode
subclass and returns itself the object is a safe string type.

This is an updated patch based on jbalogh and ivank patches.
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.
Fixed #7603 -- Added a 'scheme' property to the HttpRequest object

`HttpRequest.scheme` is `https` if `settings.SECURE_PROXY_SSL_HEADER` is
appropriately set and falls back to `HttpRequest._get_scheme()` (a hook
for subclasses to implement) otherwise.

`WSGIRequest._get_scheme()` makes use of the `wsgi.url_scheme` WSGI
environ variable to determine the request scheme.

`HttpRequest.is_secure()` simply checks if `HttpRequest.scheme` is
`https`.

This provides a way to check the current scheme in templates, for example.
It also allows us to deal with other schemes.

Thanks nslater for the suggestion.
Fixed #21263 -- Fixed issue with override_settings in inherited classes

When both parent and child classes are decorated with override_settings,
child class settings should take precedence.
Thanks Sephi for the report and Marc Tamlyn for the review.
Improvement on InlineFormSet doc, refs #21006
Merge pull request #1751 from tmaster/ticket_21006_improvement

Improvement on InlineFormSet doc, refs #21006
Updated admin alert colors

refs #21220.
Moved misplaced import in backends init