Split ZeroNet into multiple parts

**Is your feature request related to a problem? Please describe.**

ZeroNet source code is currently pretty huge. It's hard to only use only one part because all parts are very connected with each other.

This is also related to #1289, #2062 and some other comments on GitHub and ZeroTalk.

**Describe the solution you'd like**

ZeroNet source code should be split into multiple packages. That packages should be for core (communication between peers and related protocol things), user-related things (configuration, saving sites...), WebSocket server (for [integration with other software](https://gist.github.com/filips123/14d5979db24b20cdcfd40e3faacf1033)), UI server (for complete ZeroNet like it is today), command-line interface and additional plugins. And maybe also some additional packages if needed.

This would allow easier development of ZeroNet itself and also easier [integration with other software](https://github.com/HelloZeroNet/ZeroNet/issues/1289).

Also, there should be proper documentation of ZeroNet protocol so it would be easier to make ZeroNet implementations in other languages.

まず関連する issue #1289 と #2062 を読み、次にリンクされている ZeroTalk の議論と現在の ZeroNet のソースレイアウトを確認してください。完了には、core、user、WebSocket、UI、CLI、plugin パッケージへの分割について合意され、さらに他の言語での実装向けのプロトコルドキュメントが用意されていることが必要です。

索引モデルが issue の本文から書いたものです。