Resolve outstanding issues with images

[struan]

The plan for this is to

• - Allow images to be uploaded over the API and stored locally - can't directly upload image data #214
• - Switch to using the Open North image proxy - Consider changing image proxying to use https://github.com/opennorth/image-proxy #238
• - Proxy externally hosted images through the image proxy - Handle image thumbnails properly #527

Images to be stored in the filesystem according to the same system as the front end does.

May need to add in some functionality from our existing proxy to the Open North one. Also, note to investigate how this caches especially WRT to the externally hosted images.

Also, need to correctly deal with existing documents that only have an image attribute and not an images attribute - #431

[struan]

Fixed in df4d837 and a1fd8df