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
No, you set explicitly set to False. If you set it to True, then you get the clear button. To fix the test, change the value to False to match the code change you made below.

But a better fix would be to make the setting editor never show the clear button for read-only fields. Then you wouldn't have to manipulate the value of explicitly set just to hide the clear button.