Resolve "Job deletion message on job list redirection"

I fixed the bug in two DeleteView views. The SuccessMessageMixin doesn't work with DeleteView views because it's hooked to the form_valid method which doesn't exist in this particular view. This is explained in the issue #21926 if the Django project.

I took the opportunity to rewrite the other success or error messages in the other views in order to have something a bit clearer and easier to read.

Closes #158 (closed)