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 it would seem I can't make read-only fields not clearable. I could try adding an additional dictionary item like `non_clearable`, but I'm not certain this is the correct approach. More pointedly, we're sticking something we're doing our damndest to make untouchable into a property called `editable_metadata_fields`.

If we're going to do it this way, we have a few things that would need to be done first, as I understand it:

1. A PR to XBlock will need to be made creating a kwarg for field Field named something like 'info', used to mark fields which should show up as information only in the studio.
2. We add a flag in `_create_metadata_editor_info` to take advantage of this fact, named `non_clearable`.
3. We update the templates to recognize and follow this `non_clearable` flag.
4. We update the view that handles saving data to complain if a user tries to clear a `non_clearable` field.

This is a lot of work just to show the ID of this block. Is there some other way this can be done? Perhaps instead of having the ID stored in the editable fields, we could place it where it was before, except instead of just leaving it having out, have a little `more info` button that expands and displays the ID there.

If that sounds good, the question would be if we have any other place in the code where something similar is implemented so I can make sure the styling is similar.