Auto-renew Let's Encrypt

Right now a user has to manually add a command to their system cron in order to renew LE certificates that we request. This is undesirable, and we should instead offer a way for GitLab itself to auto renew these.

Since we do not want to alter the system cron, we could consider packaging our own cron within Omnibus. One option that has been discussed is embedding a go based cron service (https://github.com/robfig/cron) or Jobber.

This would allow us to schedule our own jobs internal within GitLab, such as the renewal.