Session Expiry

Add a "sessionMaxAge" key to the site config, containing an integer (seconds).

Warp only updates the session age if it's older than a granularity, which is 10 seconds by default, to avoid unnecessary DB hits. So the real max age is the configured value plus the granularity.

Expired sessions are deleted only when hit, so a cron job removing old ones once a day or so may be a good idea.