Public messages can be sent to users, and it causes problems

# The problem

I can send a public message to another user via the CLI, and their client will show a new message badge, but it will not display the message in the GUI. For example:

`keybase chat send --public dxb,chris "Hey, what's up?"`

The above command will send a message from my account (dxb) to chris' account and he will get an orange unread message badge in his client but there will be no other indication of where the message came from or how to read it. In order to clear the badge he would have to do:

`keybase chat read --public dxb,chris`

but there's no easy way to know that the message was from me or that it was public.

# Suggestion

I would suggest completely disabling public messages aimed at another account as anyone can read these messages. If there's a need for this, maybe it can be done with a public message to yourself and an `@mention` of the other person.

Another fix would be to somehow expose public messages in the GUI, and probably some other options I'm not thinking of.

Start by reproducing the `keybase chat send --public dxb,chris` and `keybase chat read --public dxb,chris` flows described in the issue, then trace how public messages reach the GUI and unread-badge handling. The desired outcome is not settled: confirm whether public messages to another user should be blocked or made visible and readable in the GUI.

Written by the indexing model from the issue text.