Add inline_discussions_cohorting_default and cohorted_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:

* inline_discussions_cohorting_default: Set it to False to get the inline discussions NON-cohorted by default for a cohorted course.
* cohorted_inline_discussions: Array of id to specify inline discussion that are cohorted. ie: 
``` 
"cohorted_inline_discussions": [
        "c2293fa2538a41eca7224b8a07c3d09d"
    ],
```
The implementation is simple and doesn't affect much code. 

cc @antoviaque
Thanks Greg! I didn't know that syntax was valid. It's fixed.