Add authentication dialog

Introduce start_auth_flow, a function invoked by typing `/auth username`
in OOC. It sends an public-key authentication request to the server,
starting the entire flow.

The flow invoves two dialogs: to select the key, and to enter the
passphrase to unlock the key. For convenience, each successful unlock
also remembers the key for that username on the server, storing this
in `saved_auth.json` (I chose JSON because I wanted it to stay
human-editable; INI would be better, but it suffers from bad platform
quirks in Qt).

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