autoprint file display for some fields; hide sort,thumb, etc

Some fields don't need to be displayed or made editable.
add file path_rel to file's JSON file; might be useful
links between master and mezzanine files

Each file has a 'links' field, which is a semicolon-separated
list of associated files' path_rels.
Functions that get outgoing and incoming links for a given file.
List outgoing, incoming links on file detail page.