Fixed #21936 -- Refactored DeleteView to be compatible with SuccessMessageMixin

Refactored DeleteView to use the form related base classes and mixins, like the
other class-based generic editting views.  Added tests for the handling the
DELETE http method and ensuring the SuccessMessageMixin works. Updated DeleteView
documentation to reflect the new base classes.
The majority of the code changes were provided by @charettes.