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
added documentation on Model attributes
Merge pull request #2363 from nheyning/22019_add_objects_doc

Fixed #22019 -- added documentation on Model attributes
Two whitespace changes to appease flake8
Allowed some auth tests to be run independently
Added tests for django.contrib.admindocs

Refs #22015