Better user management

## Problem

Many users want to have more IDs on same provider (probably ZeroID).

This is currently only possible with Multiuser plugin which isn't really designed for such things. Users must know their master seeds each time they want to switch ID and their `users.json` can get bloated with many random generated accounts.

## Solution

There should be plugin built specifically for this.

When user visits ZeroHello, it should see which account is in use and all IDs that are in that account.
Users should then be able to list all other accounts that are stored with all IDs that are in them. It should be possible to switch account with click on it. Additionally, each account should have name that is provided by user.
It should also be possible to manage accounts. It should be possible to add or remove specific account from list and `users.json`. It should also be possible to rename accounts, re-order them and group them together into groups.

## Terms

With "user", I mean person using ZeroNet.
With "account", I mean ZeroNet account which consist of address and master seed.
With "ID", I mean username from providers like ZeroID.

Start by reviewing the existing Multiuser plugin, ZeroHello, and the users.json account data mentioned in the issue. Define the account and ID management flow before changing anything. Done would include named accounts, listing IDs and accounts, switching accounts, and managing account names, order, groups, additions, and removals.

索引モデルが issue の本文から書いたものです。