Add export TTL setting

Closes #1573 (closed)

I'm not sure if setting the TTL in minutes is a good idea, rather than seconds. We already have the worker_activity_timeout, license.ping_frequency and the 8 RQ timeouts in job_timeouts that all use seconds instead of minutes. I at least made the unit explicit in the code, but it will not be clear to sysadmins that this is the only setting out of 11 that uses minutes rather than seconds.