this branch completes the first step towards switching to our new
signing key: it includes that new signing key (in a different file,
so that our current documentation isn't broken), and makes it so Tails
Upgrader trusts both the old (to be used for upgrade-description
files until 1.3 included) and the new (to be used for the UDFs once
1.3.1 is released) signing keys.
Assigned to anonym, as he's the one of us who has designed the entire
procedure with me. Now, it would be good if others had a look at the
general plan in #8222 in case we've messed up something.
