Core file factory now creates object with a nid
Adds a job for managing file revisions

Also adds a file_helpers support module to make accessing fixture files
less verbose and ease the process of creating copies of fixture files in
cases where the tested task is expected to delete the file.
Add CoreFilesController#nid_update

This allows us to track file revisions that are made by a TEI user to
one of their objects.  Also adds a convenience method for taking a form
upload object, moving its file to a location that won't get unlinked by
the controller when the request ends, and translating params[:file] into
a simple absolute filepath.