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:
``` 
"cohorted_discussions": [
        "c2293fa2538a41eca7224b8a07c3d09d"
    ],
```
The implementation is simple and doesn't affect much code. 

cc @antoviaque
I already set myself the explicitly_set value to True. Otherwise,  the field is displayed as non-editable in the studio edit form, but there is a clear button on the right side.