The Tails signing key is not trusted from within Tails

See #15697 (closed).

It sounds weird that the cryptographic content embeded in a Tails image is not trusted, while we have extensive verification mechanism for the image itself. And actually, our OpenPGP verification documentation (wrongly) takes this for granted as seen in #15697 (closed).

Am I missing something?

Related issues

Related to #15697 (closed)
Related to #11785 (closed)
Related to #16327 (closed)

Original created by @sajolida on 15710 (Redmine)

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