Replace gravatar

This avatar uploader uses our existing dropzone.js support to set / unset avatars.

The main thing to note about this use of dropzone is that only the last upload will be set as the avatar, other than that it's pretty straightforward, I think. Thoughts?

![avatar uploader](http://jbrt.org/ddg/avatar.png)

![avatar admin post](http://jbrt.org/ddg/admin_post.png)

You can check it out on https://view.dukgo.com/