Fixed #21435 -- Improved error message when urlconf is empty.

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.
Fixed #22123 -- EmailValidator now also accepts IPv6 literals in addresses
Fixed #21902 -- Documented search order for list_display.
Fixed #22120 -- Documented persistent activation of languages and cleaned up language session key use
Fixed #22092 -- Documented ResolverMatch.view_name
Extra new line to make flake8 happy
Fixed #22032 -- added documentation for settings.MIGRATION_MODULES
Fixed warnings when building the documentation.
Fixed #21741 -- Fixed render_to_string to stop pushing empty dictionaries to its Context

Thanks to kezabelle for the report and original patch
and to numerodix for his improved patch.
Fixed #22113 -- changed object_name to model_name in CBV docs.

Thanks to trac user nikunj.sg for the report.
Fixed #22095 -- Enabled backward migrations for RunPython operations

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.
Fixed #21881 -- Clarify behavior of {% ssi %} template tag.

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.
Removed incorrect statement from docstring

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
3ede006fc98f7e96ae9fb997872f78635576d5f8 and FilterExpression
is instantiated from inside the compile_filter help function.
Fixed #22063 -- Added reference to NoReverseMatch from the reverse() documentation