Encapsulate solr doc fetching

Noticed that we had SolrDocument.new(ActiveFedora::SolrService.query..." sprinkled throughout code where we had substituted fetching a SolrDocument object for accessing Fedora directly.  This seemed messy plus fragile, so I went ahead and added a method to application_controller that handled that logic.  Another benefit of doing this is that an ActiveFedora::ObjectNotFoundError is reliably raised if the query for creating the document object comes back empty handed.  Also did some incidental cleanup of instance variables we didn't appear to use or need.