Clarify behaviour of None in checkbox group => should clear up all data including follow ups

The "None" option of a CheckboxGroupSentiment component is slightly ambiguous, as the UX might not completely match the data model:

From UX standpoint:
- when checking "None", other elements are unchecked
- but currently, followups are not deleted
- checking other elements is disabled, but not follow up question
- you can't add a sentiment to None


From data model standpoint:
- elements are tied to a list of followups, that are just strings
- sentiment is just one kind of followup, comments could be another or whatever we want

This leads to scenarios like this, where you have a positive opinion of buttons, yet said you used none of those elements.


The current behaviour might be the right one, but I feel like treating sentiment as a generic followup may be weird.
But perhaps "None" should be handled more "aggressively" and remove all follow-ups?