This PR changes the default behaviour of the discussions experience by making the previous "unit-level visibility" the default mechanism for configuring discussions. Prior to this PR, under the new discussions configuration experience, all units would automatically get assigned a discussion topic and have discussions enabled for them (other than units in graded or exam subsections). However, if authors wanted they could enabled a custom visibility mode which would allow toggling discussions on or off on a per-unit level. This PR makes this custom visibility mode the standard behaviour (and eventually, only behaviour) and enables discussion for all units by default. This replicates the behaviour that already existed, however, now gives authors control over disabling discussions for individual units by default. It also removes the ability to disable discussions for all units (while still keeping course-wide discussions) enabled.
CMS
===
This directory contains code relating to the Open edX Content Management System ("CMS"). It allows learning content to be created, edited, versioned, and eventually published to the `Open edX Learning Mangement System <../lms>`_ ("LMS"). The main user-facing application that CMS powers is the `Open edX Studio <https://edx.readthedocs.io/projects/open-edx-building-and-running-a-course/en/latest/getting_started/CA_get_started_Studio.html#>`_
See also
--------
* `CMS vs Studio terminology <../docs/decisions/0013-cms-vs-studio.rst>`_
* `CMS vs LMS boundaries <../docs/decisions/0005-studio-lms-subdomain-boundaries.rst>`_