[web] Move request handling logic into cubicweb application. (closes #2200684) We improve http status handling in the process: ``application.publish`` have been renamed to ``application.handle`` to better reflect it's roles. The request object gain a status_out attribute to convey the HTTP status of the response. WSGI and etwist code have been updated. Exception gain status attribute

undo_actions = config.cfgfile_parser.get('MAIN', 'undo-support', False)
config.global_set_option('undo-enabled', bool(undo_actions))