Sprockets-ify fabric from source files instead of from pre-built

This means, not using a pre-compiled build of fabric.js but instead using a sprockets **manifest** to compile everything as needed.

This also allows for customisation of what parts of fabric to pull in.

See the [fabric.js manifest](https://github.com/davidjrice/fabric-rails/blob/sprockets/vendor/assets/javascripts/fabric.js)