Centralize download URL generation

URLs for download (using nxfile or nxbigfile) are generated manually in a lot of places in Nuxeo. This should be centralized in order to be made pluggable.

At the same time, there is no meaningful distinction between nxfile and nxbigfile these days (the advantage of nxfile was to avoid getting a new session but that hasn't been a problem for a long time), so we make them both use the same servlet download mechanism and not Seam.