Picture for architect thumbnails

Bit of a kludge, but added in auto-thumbnail for entities. It works by loading **all expressions** and looking through the the `depictsArchitect` and `depictsBuilding` properties for a match for the entity. This should really be taken care of by the server, but it automatically adding the `preferredImage` property whenever an expression is uploaded that depicts and entity, and that entity does not have a `preferredImage` already.