PaginationMixin: Get a controlled paginate_by value from the querystring or use a default.
What the docstring says. To not use some functionality, e.g. managing the value in the User's Profile model, delete the corresponding lines (when getting the page_size and when saving it. Add the Mixin before the View class. e.g.: `class ItemList(PaginationMixin, generic.ListView):`
- pagination
- paginator
- mixin