Prevent user deletion upon questionnaire deletion #256

Currently when a user deletes a questionnaire within Dashboard they are prompted that the respective user account will be deleted as well. With 2.0 this should not be the case if someone (such as a volunteer or organizer) were to accidentally create a questionnaire and didn't need it.

Will require:

• Removal of user.destroy in questionnaire.destroy
• Verbiage changes to questionnaires/edit.html.haml
• Test adjustments in tests/controllers/manage/questionnaires_controller_test.rb