Update the deb.tails.boum.org archive signing key

It expires on 2019-05-01 (spotted by our test suite).

High prio as this makes the output of our CI less useful.

Feature Branch: bugfix/16420-update-tails-apt-key

Original created by @intrigeri on 16420 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information