Download logic refactoring

• DownloadLinks record now removed on expiry (unless static)
• zip_file and and zip_contents no longer used
• Downloads now simplified - cannot get a "someone else is downloading)
• Downloads exist or they do not. If they do the user is presented with the file.
• missign sdfs directroy rename
• supression of sshtunnel connection errors
• Dynamic download file timeout now 90 minutes