Reduce bandwidth usage of images by converting to either JSON or PNG

[gitchly]

Fabric.js has a feature which allows us to serialize a canvas to a JSON object. In some cases, this may be a lot smaller than the base64 PNG representation of the image, and it might be worth sending this over the network in order to save bandwidth.

See http://fabricjs.com/docs/fabric.Canvas.html#toJSON

This would require changing both the client and server-side javascript to implement.

Client side changes:

• Client would need to convert image to both PNG and JSON, compare the sizes, and send the smallest one (shortest string).
• Upon receiving image data from the server, the client would need to parse whether it's a PNG or JSON and display it appropriately

Server side changes:

• Server image validation would need to be changed to account for the different types of data