clean up CourseListView to have fewer, more standard superclasses

[btbonval]

CourseListView subclasses ListView, ModelFormMixin, and ProcessFormView. This isn't any particular Django recipe I've seen in any docs. It seems pretty non-standard, and it makes the functionality difficult to discern. Plus modifying it can be difficult since the usual fixes found in the usual documentation assume the usual Django workflows. This is a Django workflow, clearly, but it is not a well documented usual workflow.

Ideally this can be simplified to be something more Django-like.

[btbonval]

This is being done (indeed has been done) under the auspice of #294