LDAP user sync

[baryluk]

Right now iris-sync-targets only supports syncing mailing lists / groups from LDAP.

For many use cases, it does not make sense to use oncall, and instead use LDAP for users. This is so it is easy to setup target user, or for iris-relay to know about the user (i.e. from slack).

I would like to work on this, for our setup (we do not use oncall at the moment, because we instead route alerts to various shared slack channels, not persons; and we do have a lot of time specific requirements, i.e. weekdays/weekends, working hours/non-working hours), but any input from other users and maintainers, would be useful. I am committed to implementing this.

I think it should be synced from ldap to mysql periodically, so even if ldap is down or unreachable, full alerting / escalation stack works.