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.

Start by reading related issues #1289 and #2062, then review the linked ZeroTalk discussion and the current ZeroNet source layout. Done would require an agreed split into core, user, WebSocket, UI, CLI, and plugin packages, plus protocol documentation for implementations in other languages.

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