[Feature] Data synced on multiple devices with same user ID

Added content to this comment on 12/12/2018.

Yesterday I thought about data sync on multiple devices with same ID, then I realized how it is possible.

# How ZeroSync can work

One device is set as Master, and creates a different UI IP server where other ZeroNet instances from other devices can connect (with privatekey authentication). From any device, users connected to same network and having ZN authentication, can access the Master's ZeroNet (download new zites, use the ID, sign/publish zites, sign to post/comment, etc). The detail is that the data is being remotely accessed, but not downloaded to the device to be used on-go, but:
To sync data after use: If the PC (for example) is the Master, and user needs to take the phone on-go, he presses a button to pull all Master data to the phone, or to any or all other devices. All this with the same ID. Then user can leave the area of the wireless network, and the data is already portably downloaded on the device.

The issue proposes a Master device, authenticated connections, and explicit data transfer to other devices, but names no files, tests, or entry points. Begin by mapping the existing authentication and data-storage flows, then define the synchronization design and validation needed for portable access with one user ID.

索引モデルが issue の本文から書いたものです。