Completed questionnaire should not be editable

[ldodds]

When a user has completed a questionnaire, if they hack the URL to go back to the questionnaire page, then the answers are still editable.

This means a user can update their answers, but can't then re-complete their questionnaire.

Originally the intended behaviour was that once complete a questionnaire shouldn't be editable, to avoid scores being revised retrospectively. But there are use cases where it might be useful/valid to do this. So a fix for this bug might be to re-evaluate that decision and revise the other application logic accordingly. (Currently there's no "rescoring" facility)