Fix error when an imageUploader isn't provided.

I didn't think this through very well before and hoped it would just
work so I wouldn't have to push uglier code like this.

Test Plan:
* Load perseus one without imageUploader specified in the perseus
  config. Drag an image into a textarea and the page is redirected.
* Reload with imageUploader specified and drag and drop works as before,
  uploading an image.

Auditors: alex