[webrequest] Lazy-load search_state - The update_search_state function will hit the session data only if __mode is present in the form. - set_search_state keep the same semantics - Direct access to search_state in the session data is changed by an access to the new property. Closes #4875761

cubicweb-web: embedded-javascript-library usr/share/cubicweb/cubes/shared/data/jquery.js