Fixed several flake8 errors
Fixed #14481 -- Documented implicit "through" class created by ManyToManyField.

Thanks to jonathanmorgan for the report and initial patch.
Fixed #22778 -- Added a model Meta option to define default_related_name.

Thanks jorgecarleitao and mmardini for reviews.
Fixed #22859 -- Improved crossDomain technique in CSRF example.

Thanks flisky for the report.
Removed a CSRF example for jQuery < 1.5.