add login option: stay_logged_in

As proposed in issue #354; it adds an attribute max_age
to mediagoblin.tools.session.Session  that is passed to
response.set_cookie;  max_age is set to 30  days if the
checkbox is selected
save “stay_logged_in” in the session

Since sessions are rebuilt, e.g. when you try to post a blank
comment and therefore receive an error message, the session will
be overwritten without the old max_age.
updated to new render_divs macro