… type in course

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.

(cherry picked from commit cbddd4e)