feat: allow hiding question count on the course outline page

[Agrendalath]

Description

The CourseBlockSerializer serializer appends the "(N Questions)" suffix for the subsection name, so there is no reasonable way to hide this information. This PR introduces a Waffle flag that omits this suffix.

Testing instructions

1. Create a course with Problem XBlocks.
2. Check that you see the "(N Questions)" suffix on the Course Outline page.
3. Create the course_home.disable_sequence_question_count Waffle flag.
4. Check that the suffix is no longer displayed on the Course Outline Page.

Deadline

"None"

Other information

I believe the "proper" fix here is to move responsibility for rendering this text to the Learning MFE. However, changing the default API response would require creating a new endpoint version, and the course_home API is currently not versioned, so the required effort would increase by an order of magnitude. Therefore, I decided to take a less risky approach.

Private-ref: BB-7413

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