Remove `POST /images` in favor of `POST /uploads`

* Uploads takes an image URL and backgrounds the image creation
* Allows quicker upload feedback
Don't expose Image creation endpoint

Image creation requires talking to S3. Talking to an external site during
client-server communication is a no-no :tm:. Instead, create an `Upload` via
`POST /uploads` with the File URL. The upload endpoint queues up a DelayedJob to
do the file moving and Image creation.