Update course members sections

[abeeto]

• using action decorator to set up endpoint: api/v1/course-members/{id}/update-sections/
• now validating the request.data and checks if:
  • is a list of numbers
  • does not accept a list of sections that are not in the course of the course-member
• using transaction.atomic and action decorator to configure a endpoint to update course-member
• the sections field is being validated through the use of a serializer made for this endpoint called UpdateStudentsSectionRequest in views/course_member.py
• endpoint returns the same new student once a valid set of ids are returned.

[abeeto]

Sorry about the previous PR, it was late at night and I got way too greedy with GPT/copilot. Hope the changes make more sense now.

[Opeyem1a]

this isn't the goal of your task, but idk this endpoint seems kinda messy. I would prefer to see an actual schema of what the response object looks like. An example is something like:

{
    "data": ...the new serialized course member after update,
    "error": ...an error object if there is one
}

[Opeyem1a]

fields that start with _ usually aren't meant to be accessed directly. Are you sure this is the correct pattern here?

I feel like there is a clean() method on serializers that returns the validated data