Working with the Clean-param section

[KitKat-ru]

Users there is a need to manage the Clean-param section. Since this parameter only supports Yandex, It was not possible to find a working solution for the administration of this field. There is no such functionality in the current implementation of `django-robots'.

Example of changes:

1. Templates - add a section to the generated file robot.txt
2. Views - Add a method to get all clean_param objects from the Rule model.
3. Model - add the clean_param field associated with M2M to the new CleanParam model.
4. Model - create the CleanParam (example-name).
5. Add the appropriate changes to the AdminForm.

Perhaps these changes can help library users implement a better SEO site setup.