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.
- Moving xmodule folder to root as we're dissolving sub-projects of common folder in edx-platform
- More info: https://openedx.atlassian.net/browse/BOM-2579
- -e common/lib/xmodule has been removed from the requirements as xmodule has itself become the part of edx-platform and not being installed through requirements
- The test files common/lib/xmodule/test_files/ have been removed as they are not being used anymore
