Improve systemd unit

• Pull in network-online.target and order after nss-lookup.target. Quassel needs a working Internet connection.
• Add ExecReload to reload the certificate.
• Do not activate postgresql.service, simply order the unit after it.
• Fix PIDFile to get rid of a message about using a legacy directory.

Fixes #4 Fixes https://bugs.archlinux.org/task/60796