Pin root of trust in chrome => native upgrade flow

[jpouellet]

While upgrading from the chrome app to native signal-desktop, the "Debian-based Linux" install instructions recommend:

curl -s https://updates.signal.org/desktop/apt/keys.asc | sudo apt-key add -
echo "deb [arch=amd64] https://updates.signal.org/desktop/apt xenial main" | sudo tee -a /etc/apt/sources.list.d/signal-xenial.list
sudo apt update && sudo apt install signal-desktop
/opt/Signal/signal-desktop --import

(defined in background.html#L599-L602)

This relies on SSL to bootstrap trust in the package signing key from scratch, but we already have a trusted chrome extension from which we could bootstrap trust in that key.

It would be nice to somehow automatically pin the correct fingerprint via the install instructions, or at least provide it in a user-visible comment below the instructions.