The new mechanism for marking that a unit has discussions is to use the
discussion_enabled flag instead of adding a discussion block. This change
adds code that is run during the course rerun process to mark any existing
units that have a discussion block as discussible using the new mechanism.
It doesn't touch the existing discussion blocks.
If the new discussions configuration experience is globally enabled, this
will also switch from the legacy provider to the new provider. It analyses
the course for any discussion blocks that have been added to graded
subsections, and if that is the case, it also automatically enables
discussions in graded subsections (which are otherwise disabled for new
courses by default).
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