improve js on episode form page

Before, every updated caused the whole media file table to be rendered
again. Now, only the affected row is rerendered.

Also, I wanted to trigger the auphonic logic more intelligently. So now
I trigger `.trigger('mediaFileHasUpdated', [url]);` on the slug field.
The auphonic module watches it and only reacts when the url in question
ends with "json" instead of every file change.