Respect question enabled

InquistionQuestion.enabled was added in r78654 / 8a05894 but the loading of the bindings was never updated to respect it.

Now calling code can use `InquisitionInquisition->visible_question_bindings` and `InquisitionResponse->visible_question_values` to access only visible questions, if the code should pay attention to question visibility.

Question visibility is not shown in the admin tools by default, and should be used with caution. For example, if enough of a graded inquisition changes over a period of time, old responses can suddenly switch from passed to failed, unless great care is used to only calculate it based on visible questions at the time the response was saved.
Add `false_content` to this field and remove the `?`