Changes Image#image_file method to Image#image_url to be consistent with Video#video_url.
Changes solr field from "image_path" to "image_url" to be consistent with "video_url".
Adds OpenvaultAsset#thumbnail_url, with intention that it be overwritten for models that extend OpenvaultAsset.
Adds "thumbnail" as dynamic solr field to solr document returned by OpenvaultAsset#to_solr