Implement the proper loading of the link to attribute.

* Add a model PostImage (name TBD) to be used for images inserted in a post. The model supports both internal (attachments) and external images.
* Incorporate logic needed to determine the type of link (file, post, custom, or none).
* Switch to extending Settings.AttachmentDisplay to pickup some additional logic.
* Make attachment and the dfd reference a property of the PostImage model.