Make Tails Installer installable through Ubuntu Software

As noted in #11004-note_4, the installation process in Ubuntu has changed in 16.04 with the introduction of their new “Ubuntu Software” (branded version of GNOME Software). My rough notes are:

• Start Ubuntu 16.04 live.
• Choose Try Ubuntu.
• Start Software & Updates.
• Check universe.
• Other Software → Add… → ppa://tails-team/tails-installer.
• Close, Reload.
• Open a terminal.
• Execute `gnome-software`.

But Tails Installer won’t show up in Ubuntu Software until we have AppStream metadata. See #11569 (closed).

Start reverting e74b5eeb36 to hopefully rescue some strings and translations from there.

Also double-check if GNOME Software is in Stretch by default and if so, consider replacing Synaptic with GNOME Software for all distros. Debian would still need a custom section to configure backports.

Subtasks

• #11004 (closed)
• #11404 (closed)
• #11570 (closed)

Related issues

• Related to #11561 (closed)
• Related to #12695 (closed)
• Related to #12688 (closed)
• Related to #15396
• Blocked by #11569 (closed)
• Blocks #16141 (closed)

Original created by @sajolida on 11378 (Redmine)