Sort out packaging Git repo for Tails Installer in Debian/Ubuntu

On the one hand the package is team maintained. On the other hand, Vcs-* fields point to a repository where nobody on the packaging team (but intrigeri) has write access to. This is inconsistent, and won’t work in practice.

Feature Branch: 451f:feature/10666+tails-installer-doc

Parent Task: #8549 (closed)

Original created by @intrigeri on 10666 (Redmine)