Allow a teacher/admin to delete a lesson

[amellie]

Dependency

Depends on #27

Story

As a Teacher or an Admin
I Want to delete an existing lesson
So That I can keep the lessons up to date

Acceptance Criteria

• The delete capability should only be accessible by a teacher/admin
• Deletion can be done from the lesson list page created in Allow a teacher/admin to see the list of lessons with controls #27, by clicking the Delete button. The lesson list page should be updated to allow this control
• When a lesson is deleted, ALL of its associated submissions should be deleted. In the database level, deleting a lesson should result in the automatic deletion of the submissions. This can be done if you set DELETE CASCADE on the lesson table as explained here
• The lesson documents and submission documents should also be deleted permanently from the server
• Ensure that users can confirm Yes/No if they REALLY want to delete the lesson (after clicking the Delete button). Please also mention that deleting this lesson will result in the deletion of all of its associated submissions and the action cannot be reversed
• Upon deletion, the user should be redirected to the current lesson list page with a message "Lesson has been deleted" (if successful) or show an error message if unsuccessful