It'd be nice if our entity pages informed the user of possible duplicates before they submitted the form. This'd be a prerequisite for moving the editing pages to React and away from FormHandler.

I think it'd be sufficient to have the JS (and HTML5 'required' attribute, where that exists) block submission, and to move the existing FormHandler validation to the actual edit class initialization methods, since I assume we want to enforce the disambiguation rules no matter how the edits are created (e.g. if the pages were changed to use an editable web service later).