Make the questionnaire checkbox hint "Multiple choices are selectable" togglable on/off

The teacher should be able to decide if questionnaire checkbox hints show the hint "Multiple choices are selectable." (on/off toggling for each question, i.e., pick-any RST directive).

The wording of the hint must be improved: “You may choose multiple options.”

The hint must not be removed completely since some teachers have valid use cases for it (e.g., Programming 1).

Original description when I was going to remove the whole hint completely:

I have never liked that message. It should be obvious that multiple checkboxes can be selected when the question has checkboxes instead of radio buttons. In addition, the hint reveals that the correct answer contains more than one checkbox, which does not make sense to me. The teacher may make questions where only one checkbox is correct, but it is part of the challenge to understand which and how many alternatives are correct. The hint could be displayed all the time if we really think that the student does not know how checkboxes work, but I would prefer to remove the whole hint "Multiple choices are selectable".

Originally posted by @Mankro in #93 (comment)

mooc-grader/access/types/forms.py

Line 643 in db5135b

if t == "checkbox" and correct_count > 1:

@jsorva said in the other issue #93:
#93 (comment)

To summarize, it is important that the teacher may enable or disable this hint. The wording can be improved: “You may choose multiple options.”