Temporary fix guide featured_images

# Why?

 * I got a bug report from production that is caused when people attempt to update a guide via the API using a relative URL instead of an absolute URL

# Temporary fix

 * Since featured_images aren't done yet, I am stubbing it out to empty-pot.png for now

# Long term fix

 * It might be time for us to start using [RestPack Serializer](https://github.com/RestPack/restpack_serializer) and [Mutations](https://github.com/cypriss/mutations) to better handle these situations.