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.