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
@cahrens I'm picking up this task now that @aboudreault has moved on. I was looking up the source of this problem, and found the following:

                // If the model has property `non editable` equals `true`,
                // the field is disabled, but user is able to clear it.
                if (this.model.get('non_editable')) {
                    this.$el.find('#' + this.uniqueId)
                        .prop('readonly', true)
                        .addClass('is-disabled');
                }
            },

It's in `cms/static/js/views/metadata.js`, and appears to imply that this seeming oversight is a deliberate design choice. Which makes me wonder if changing this would actually have unfortunate and unintended consequences. Do you know why this decision was made, and do you have any idea what it might break?