Support faster deletes of sections

I've optimized deletes in the course outline view so that they behave the same as adds now. It used to refresh the entire course when a section was deleted, but that was unnecessary and very slow. I've changed it to mirror the similar optimization that I did for adds. 

Now deleting a section will just remove the relevant child view without a refresh, except in the case of deleting the final section. The latter case requires a full course view refresh in order to get the "You have no content in your course" message.

I've added a new unit test for deleting the last section so now there are tests for both situations.

@cahrens @dan-f Please review.