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

[eric-burel]

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?

[SachaG]

I think "none" should probably clear all follow-ups as well.