Discussion: Add cohorted_threads_privacy cohort setting 

This PR adds **cohorted_threads_privacy** cohort setting: a user that is not in any cohort will only see non-cohorted threads.

This is a need for a client. By default, for a cohorted course, if a user that is **not** in a cohort group accesses the discussion tab, he will see all threads, even those that have been posted in a cohort. The  **cohorted_threads_privacy**  setting allow you to change that behavior. If you set it to 'cohort-only', a user not in a cohort will only see non-cohorted threads.

```
{
    "cohorted_threads_privacy": "cohort-only",
    "cohorted": true
}
```

@jimabramson @gwprice