Upgrade to Stretch 9.1 in Tails 3.1

Originally created by @intrigeri on #13178 (Redmine)

Usually we don’t update to Debian stable point-releases in a Tails point-release, but a Debian r1 (first point-release after the initial stable release) generally brings tons of bugfixes that we might want. The RM for 3.1 (bertagaz) agreed to consider pulling these fixes.

Here’s what it will include:

https://release.debian.org/proposed-updates/stable.html

Let’s set up a branch with stretch-proposed-updates enabled, so we can evaluate the impact.

Feature Branch: feature/13178-stretch-9.1

Related issues

• Related to #12543 (closed)
• Blocks #13234 (closed)