Rewrite Ubuntu instructions to install Tails Installer from Synaptic

The new software center “Ubuntu Software” doesn’t work for installing Tails Installer PPA, with a similar caching bug as with Ubuntu Software Center but no more workaround as update-apt-xapian-index has disappeared. I propose that we instead rewrite these instructions to rely on Synaptic which is more reliable, even if it will make the instructions more complex as Synaptic is not always installed on Ubuntu.

Parent Task: #11378 (closed)

Related issues

Related to #11561 (closed)

Original created by @sajolida on 11404 (Redmine)

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