Fix error when an imageUploader isn't provided.
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

alexristich/perseus,chirilo/perseus,huangins/perseus,abe4mvp/perseus,abe4mvp/perseus,ariabuckles/perseus,learningequality/perseus,junyiacademy/perseus,daukantas/perseus,sachgits/perseus,junyiacademy/perseus,ariabuckles/perseus,alexristich/perseus,sachgits/perseus,iamchenxin/perseus,ariabuckles/perseus,learningequality/perseus,huangins/perseus,iamchenxin/perseus,daukantas/perseus,chirilo/perseus,ariabuckles/perseus