feat: subsection list

[navinkarkera]

Description: This PR adds subsection component which enables UI to display subsections under each section. It also adds functionality for adding new subsections, duplicating, deleting and publishing subsections. We also try to reuse section list as much as possible.

Private-ref: https://tasks.opencraft.com/browse/BB-8221

Dependents on:

• feat: create DRF endpoint to get course index context openedx/edx-platform#33667
• feat: course outline page openedx/frontend-app-authoring#694

Testing instructions:

• Setup this repo as described in the readme.
• Checkout branch from feat: create DRF endpoint to get course index context openedx/edx-platform#33667 and run make cms-up in devstack.
• Open any course outline by using this url format: http://localhost:2001/course/course-v1:edX+DemoX+Demo_Course/

Screenshot:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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Additional details and impacted files

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[CefBoud]

Great job on this, @navinkarkera !