Add cohorted_threads_privacy cohort setting

Add cohorted_threads_privacy cohort setting: a user that is not in any cohort will only see non-cohorted threads.
It might be clearer/more reusable to have a `def cohorted_threads_privacy(course_key)` function that returns the setting value, instead of checking for it inside the function. This way it matches the setting name/value, and allows to have something more readable below (cf other comment).