Fix drag-and-drop redordering for question options

[sanason]

See linked issue for description of the problem.

This PR:

• Adds backend validation to enforce that a reordering must be completely specified - it must assign a unique position to each of a question's options. This prevents ambiguous orderings where, for instance, multiple options have been assigned to position 1.
• Fixes the front-end code to actually perform drag-and-drop for question options.
• Initialize event listeners so that drag-and-drop still works after adding or editing a question, without requiring a page reload.
• Added controller and feature specs.

Manual tests performed:

• Load the Questions tab for the Kitchen Sink form and test that each of the following question types supports drag-and-drop reordering of question options: radio buttons, dropdown, checkbox, combo box.
• Drag-and-drop changes are persisted to the database.
• Within a question option, it is still possible to click on the delete button (to delete the option) and the option label (to update the option text and value).
• Question option is draggable immediately after being added to an existing question.
• Question option is draggable immediately after being added to a new question.
• "Other" option is draggable immediately after being added.
• Options are draggable after editing a question and then either Saving or Cancelling.
• Drag and drop reordering of questions still works.