[MilosKozak]

Follow-up to #2638. The matching code is in AndroidAPS dev already, so this can be merged now.

The master now signs the settings it puts in Nightscout for its clients (the settings, the client list and the "master is alive" sign). A client takes them only when the signature is the master's, so another client's Nightscout token cannot change them.

What changes, all on Master ↔ Client control:

• Security notes: a new point, "The master signs what it shares", and how a client learns the master's key (when it pairs, or from the QR code for a phone that only reads Nightscout).
• Troubleshooting: a new row for the NSClient log message "✕ SETTINGS … refused" and what to do: a paired client takes a new key by itself, and a phone that only reads Nightscout needs the master's QR code again.

utils/qualitycheck.py --lang EN passes.

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