Submit 'featured_image' as URL to API

# What's New?

 * API users may now submit a featured image to the API via an HTTP URL (Provided in the `featured_image` attribute).
 * Server will download URL and move it to S3. Future GET requests will reference our copy of the image on S3.

# What's Next?

 * Need to work with @roryaronson to get the S3 credentials onto production (I tested locally with my personal S3).
 * Need to setup a `temp` folder for API users to store.