As a teacher
I want to be able to delete a section in the course index
so I can clean the content up

As per current behavior, deleting a section deletes from the database:

• The section itself
• Information about section availability
• All activities included in the section, and information related to those activities.

(forget about the rest of the options for now)

Prototype: https://www.figma.com/proto/OfCvqK069Oz6Fvqq2xcZWI/Create-a-course?node-id=3285%3A38308&scaling=scale-down

Acceptance criteria:

1. User needs permissions to delete sections.
2. User needs to confirm the action.
3. The section will be removed in the course index and course main content.
4. All activities in the section will be removed in the course index and course main content.
5. Section 0 couldn't be deleted via UI.
6. Some activity types would require asynchronous deletion. We will keep current behavior in those cases.