On my system, Vaultwarden fails on boot with an AddrNotAvailable error,
presumably because `services.vaultwarden.config.ROCKET_ADDRESS` is not
available yet.
Here's an example of the startup error in journalctl:
```
Jul 26 21:56:48 banana vaultwarden[2514]: Error: Rocket.
Jul 26 21:56:48 banana vaultwarden[2514]: [CAUSE] Bind(
Jul 26 21:56:48 banana vaultwarden[2514]: Os {
Jul 26 21:56:48 banana vaultwarden[2514]: code: 99,
Jul 26 21:56:48 banana vaultwarden[2514]: kind: AddrNotAvailable,
Jul 26 21:56:48 banana vaultwarden[2514]: message: "Cannot assign requested address",
Jul 26 21:56:48 banana vaultwarden[2514]: },
Jul 26 21:56:48 banana vaultwarden[2514]: )
```
According to systemd docs [1], `network.target` has little meaning
during startup, but `network-online.target` actually waits until an IP
address is available. The docs recommend putting `network-online.target`
in both `After=` and `Wants=`.
[1]: https://www.freedesktop.org/wiki/Software/systemd/NetworkTarget/
04d1756118