mediaStyleSelector javascript doesn't URL encode fields object

I was having some issues where previously embedded media items wouldn't open the style popup with the previous values. The request to `/media/{id}/format-form` to retrieve the form was always returning the default form. I found that fields containing a ampersands ('&') or a non-breaking spaces ( ) were causing this since they weren't being properly escaped before being included in the url.

The field data object is only being stringified but not escaped as a url component.