Add warning when submitting unchanged ranking

[stephancill]

Fix for #272.

When the list is unchanged and the user tries to submit, they are presented with this modal:

Something that may be an issue is that once the modal is presented, if the user presses Cancel, and they click submit again, it won't be presented again. More information on L27 of website/src/components/Survey/TaskControlsOverridable.tsx. Not sure how important this is but will required additional refactoring to fix.

[andreaskoepf]

(I removed the backend label since the proposed change is in the website code.)