Add always_cohort_inline_discussions in course settings

For a cohorted course, we needed a way to set inline discussions non-cohorted by default and select manually the discussions that need to be cohorted.  2 course settings added:

* always_cohort_inline_discussions: Set it to False to get the inline discussions NON-cohorted by default for a cohorted course. Then, you can add the inline discussions id in the cohorted_discussions array:
``` 
{
    "always_cohort_inline_discussions": false,
    "cohorted_discussions": [
        "<inline discussion id obtained from an inline discussion block>"
    ],
    "cohorted": true
}
```
The implementation is simple and doesn't affect much code. 

**Sandbox URL**: 
* LMS, with course configured to use the feature: http://sandbox2.opencraft.com/courses/TestX/TST-COHORT-1/now/about
* Studio: http://sandbox2.opencraft.com:18010/

cc @antoviaque
unless they already exist elsewhere, we should have tests to confirm that when course_settings.is_cohorted is `false`, or the user is not privileged, the dropdown is totally suppressed regardless of `is_commentable_cohorted`.