User can view dropoff in views by section and subsection

[crathbun428]

User Story
As a course author, I want to be able to see how many learners viewed pages in each section and subsection of my course, so that I have an easy way to view where the majority of my learners stop engaging with my course content.

Acceptance Criteria
Scenario: A course author can see the dropoff in views by section and subsection
Given I am a course author with X permissions
When I view the view dropoff visualization in the course engagement section of the course dashboard
Then I see the number of learners who viewed at least one page for each section/subsection of my course
And I see the number of learners who viewed all pages for each section/subsection of my course