[web] Fix expiry of anonymous sessions (closes #4154479) Things like: try: foo except: bar else: baz doesn't work very well if you expect baz to run even in the exception case. Plus, session.cnx.check() is a dbapi remnant, so drop it and just look at session.mtime.

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