[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.

missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/share/pyshared/cubicweb/skeleton/debian/rules.tmpl)