syncthing: create default group if not overridden

The following configuration generates a systemd unit that doesn't
start.
```nix
{
  services.syncthing = {
    enable = true;
    user = "my-user";
  };
}
```

It fails with
```
systemd[1]: Started Syncthing service.
systemd[6745]: syncthing.service: Failed to determine group credentials: No such process
systemd[6745]: syncthing.service: Failed at step GROUP spawning /nix/store/n1ydz3i08nqp1ajc50ycy1zribmphqc9-syncthing-1.1.4-bin/bin/syncthing: No such process
systemd[1]: syncthing.service: Main process exited, code=exited, status=216/GROUP
systemd[1]: syncthing.service: Failed with result 'exit-code'.
```

This is due to the fact that `syncthing` group (default) is not
created if the user is overridden.

Add a separate check for setting up the default group, so that
user/group are created independently.

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