login_frontend Browser: fix cookie max_age to be a relative time offset

Django's `set_cookie` handles max_age as a relative timestamp offset, i.e. it will set the cookie using `time.time() + max_age` as the timestamp. This results in a very long expire time for the cookie, which raises a `ValueError: timestamp out of range for platform time_t` on 32-bit platforms.

rgrep'd for max_age in the source, and this seemed to be the only occurrence..