[Feature] Add script for making systemd service

I would suggest that ZeroNet should come with a script to create a systemd service for ZeroNet, which would create and configure a service account for the purpose.

While trying to figure out how to do this myself, I read what somebody wrote on StackExchange:

> However, service accounts are typically created and configured by the [package] manager upon installation of the service software. So, even as an administrator you should be rarely directly concerned with the creation of service accounts.

And that made me think, "Hey, ZeroNet should do that too."

この issue ではファイル、テスト、エントリーポイントが指定されていないため、まず ZeroNet の既存の起動またはインストールスクリプトとサービスのドキュメントを見つけます。完了条件は、ZeroNet 専用のサービスアカウントと systemd サービスを作成して設定する、実用可能なスクリプトです。

索引モデルが issue の本文から書いたものです。