Feature: Splitting User files from Zite files/Data directory

I originally thought about this idea because of my ZeroNet Windows Installer. I wanted to be able to install ZeroNet globally (for all users). But the problem is that all users on Windows would share the same user files (and only being able to have one of each id provider makes this more problematic). The other options are to use the multiuser plugin, which I personally don't think is very good, or installing per user, which would mean many zites would be duplicated per user on the computer.

So, I propose that we move the user files (users.json and the other one, is it sites.json?) out of the data directory and allow specifying a directory where ZeroNet can find them in the zeroconf file. This would allow multiple users to specify their own user files. Note that I'm not sure if this is already possible.

Kinda-Problems:
* mutes.json would probably have to be global.
* Downloaded and pinned (optional) files would be global and not user-specific, but I don't think this is really much of a problem.
* Not sure how this would involve the multiuser plugin (if you'd have to change it, etc.)

Start by tracing how users.json, sites.json, and mutes.json are located under the data directory and how the zeroconf configuration is read. Then inspect the multiuser plugin and Windows installer context. Done means the user-file location can be configured separately while global data remains appropriately shared.

索引モデルが issue の本文から書いたものです。