Don't show a category in the questionnaire if the user doesn't have any questions assigned

[kevintli]

Building operators might not be assigned any questions from a particular category, in which case that category would just show a blank page in the questionnaire. We should remove these categories from the navbar and the questionnaire flow.

This may be a tricky fix because the "next" button can't just redirect you to the next numbered category; we need to find the next category that actually has questions for the user.