controller can build URL + Handles missing uploaded files

* Up till now, only controller.web was able to construct an url of the
  application using environment an wsgi stuff.
  When a script is run by cron, hence using controller.controller we might
  need URL as well. Ex: sending a mail that include the URL of the object.
  3 new attibutes are included in /etc/collorg/<db_name> to allow that new
  feature :
  _url_scheme
  _server_name
  _script_name
* When playing with developpement application (not on the production server),
  the uploaded files might be desynchronized. We deal with an absent file by
  generating a __missing_file__ URL.