fix: use a single 'provider_type' key for storing discussion provider type in course

[xitij2000]

Description

Both 'provider' and 'provider_type' have been used for storing the discussion provider type in course 'discussions_settings' field, there are some places in the code checking for 'provider' and others checking for 'provider_type', in some cases this can cause a bug where it doesn't detect the correct provider which causes discussion settings not being copied correctly when a course is cloned.

This change prioritises the provider_type setting over provider and reads provider only as a fallback. The provider setting is now made read-only just for backwards-compatibility, to avoid confusion.

Testing instructions

• Create a course structure with discussions enable or disabled in different sections. You can use this test course.
• Use the course clone API to clone the course: studio.local.openedx.io:8001/api/v1/course_runs/clone/
• Check that the discussion enabled/disabled statues is copied over.

Deadline

None

[openedx-webhooks]

Thanks for the pull request, @xitij2000!

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[0x29a]

👍

• I tested this: tried cloning a course with different discussion settings and ensured that they're preserved.
• I read through the code.