Install caff (signing-party) by default

It would be good if Tails bundled the signing-party package from Debian. At the moment I use caff(1) to do keysigning on a separate machine with TAILS, but I have to connect it to the internet every time in order to install that package. If Tails bundled it, I could keep the machine airgapped.

The package and its dependencies are about 800KB to download (on top of a fresh Tails boot) and about 23MB when installed.

Related issues

Related to #8401 (closed)
Related to #6338 (closed)
Blocked by #14570 (closed)

Original created by @infinity0 on 14581 (Redmine)

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