[Discussion] New plugin system

Moving the discussion from the PR #1430 (read from [this comment](https://github.com/HelloZeroNet/ZeroNet/pull/1430#issuecomment-420003512) onwards) to here.

*Topic*: How to write a plugin architecture for ZeroNet in such a way that everyone can install plugins from "plugin store" in sites?

*Possible choices* are to have one-click install with current plugin system, change plugin system to a more modular one, and to sandbox plugin execution in some way.

Please read the other PR before writing here.

Start by reading PR #1430, especially the linked comment and subsequent discussion, as requested in the issue. Compare the proposed one-click installation, modular plugin system, and sandboxing options with ZeroNet's current plugin system. The work is complete only when a concrete architecture and scope are agreed, rather than merely discussed.

Written by the indexing model from the issue text.