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.