Add validation for unquoted values of (Yes) and (No) in choice questions

[gregelin]

"Yes" and "No" are special keywords in YAML that map to boolean True and False.

Choice questions render "True" and "False" as only option if the choice questions uses

    text: Yes
    ...
    text: No

instead of

    text: "Yes"
    ...
    text: "No"

This should actually be made to fail our validator since a boolean is the wrong data type for the text of a choice. To get the string Yes and No, they should be wrapped in quotation marks, i.e.:

text: "Yes"
...
text: "No"