Connect new app/device endpoint

[Max-Levitskiy]

As a user, I want to have a functionality for connecting my devices and applications to the Webtree network.
It should be a similar principle as we have in the friend invite functionality.
From the new app/device we get the public key and send it to our device, which is already connected to the network. This device uses the public key to encode an invitation.
An invitation should contain:

• multiaddress of our node
• invitation code
• a public key of our device in the network

Invitation is decoded by the private key of our new device\app and initialize the connection, sending the message, encoded by public key of the connected device.