Type-check class based views from #1964

[richardebeling]

In #1964, we made a mistake with the instance variables and return types inside UserMergeSelectionView::get_success_url. Mypy didn't complain about these mistakes. I would have expected that the function signature of get_success_url from the base class is applied, but mypy doesn't do this.

To make mypy detect such errors, I think we want to type-annotate the class based views introduced in #1964 and possibly also mark them with the @override decorator.