Documents dependencies on a ubuntu system Related to #4291173

Use short-lived cubicweb sessions to let pyramid actually handle the web sessions Related to #4291173

Isolate the default handler and extend its role The handler now does the job of CubicWebPublisher.main_handle_request() and calls CubicWebPublisher.core_handle(). Instead of using config.add_notfound_view, a catchall route is defined and the handler plugged to it. Related to #4291173