* chore: Update Python requirements via pip-tools. Needed the newest
edx-django-utils version.
* refactor: Use the newly-added ReadOnlyAdminMixin from the updated module and
remove the existing implementation.
* fix: edx-platform now has a Django proxy model - CourseOutlineRegenerate.
Without this fix, the permissions to the model's admin interface cannot be
added to a group and successfully granted to users.