Fixed #22028 -- Documented that database signals aren't emitted for apps that laack a models module.

Thanks un33k for the suggestion and donjpacheco for the draft patch.

Refs #19280 and Refs #19392.

Thanks Aymeric Augustin for the suggestion.

Fixed documention typo

This allows us to ignore empty dirs or other unrelated dirs.
Thanks Sebastian Pawluś for the report and the initial patch.

By removing the 'supported' keyword from the detection methods and only relying
on a cached settings.LANGUAGES, the speed of said methods has been improved;
around 4x raw performance. This allows us to stop checking Python's incomplete
list of locales, and rely on a less restrictive regular expression for
accepting certain locales.

HTTP Accept-Language is defined as being case-insensitive, based on this fact
extra performance improvements have been made; it wouldn't make sense to
check for case differences.

Thanks skipjack at peru.com for the suggestion.

Thanks simon29 for the suggestion.

Thanks EvilDMP for the suggestion.

Thanks donjpacheco for the initial patch.

Refs #22015

Fixed #22019 -- added documentation on Model attributes

The docstring of FilterExpression said that it shouldn't be
instantiated from anywhere but the get_filters_from_token
helper function.

However, that helper function was deleted in commit
3ede006f and FilterExpression
is instantiated from inside the compile_filter help function.

When using the `parsed` option, {% ssi %} has the same limitation
as {% include %}, namely that the rendering of the included
template is done separately.

Thanks to trac user nagyv for the report and to alextreme for
the patch.

Added reversible property to RunPython so that migrations will not
refuse to reverse migrations including RunPython operations, so long as
reverse_code is set in the RunPython constructor. Included tests to
check the reversible property on RunPython and the similar RunSQL.

Thanks to trac user nikunj.sg for the report.

Thanks to kezabelle for the report and original patch
and to numerodix for his improved patch.

Fixed #22120 -- Documented persistent activation of languages and cleaned up language session key use

The new error message now hints that the most likely issue
is a circular import.

Thanks to trac user elena for the report and to
bpeschier for the original patch.

Thanks to trac user kinjal.dixit for the report.

Refs #21725.