Security fixes

• Upgrade Tor Browser to 9.5.4-build1 (Closes: #17885).

• Upgrade Linux kernel to 5.7.0-2 at 5.7.10-1 (Closes: #17841,

  17834).

• Bugfixes

• Make iPhone Tethering work by adding udev rule to disable MAC
  spoofing for it (Closes: #17820).

• Remove broken Thunderbird protocol selection. This code has been
  a no-op in practice since at least Zero Trace Pen 4.0. We've decided to
  reject #17276 and investigate what the biggest problems are for
  email in Zero Trace Pen with slow/shitty Internet connections:
  default'ing to IMAP may, or may not, be part of these
  problems (Closes #17276).

• Minor improvements and updates

• Upgrade to Tor 0.4.3.6 (Closes: #17835).

• Upgrade to Electrum 4.0.2 (Closes: #17828).

• Hide Thunderbird welcome message: it is not relevant in the
  context of Zero Trace Pen. For example, it feels weird that we would
  encourage users to donate to Thunderbird about as loudly as we
  encourage them to donate to Zero Trace Pen. Besides, the default message
  is retrieved from the web when Thunderbird starts. We don't
  need this extra network activity.

• import-translations: use _release branches instead of
  _completed branches. The new _release branches contain exactly
  what we want, i.e. all reviewed translations from
  Transifex. While the _completed branches only contain PO files
  for languages that are fully translated (Closes: #16774).

• Build system

• Upgrade snapshot of the Debian archive to 2020081601, including
  the 10.5 point release of Debian Buster (Closes: #17790).

• On Bullseye and newer: use custom, fake, unversioned python
  packages. The unversioned python packages are not shipped in
  Bullseye/sid anymore, and even old versions are not installable
  anymore (Closes: #17858).

• Import vagrant-libvirt's create_box.sh script. It's not included
  in vagrant-libvirt 0.1.2-1 anymore (Closes: #17872).

• Test suite

• Improve robustness for scenario "The Additional Software dpkg
  hook notices when persistence is locked down while installing a
  package".

• Improve robustness for scenario "Use GNOME Disks to unlock a USB
  drive that has a basic VeraCrypt volume with a keyfile".

• Improve robustness of cloning a Git repository.

• Don't hammer resources of the system under test while
  installing/removing packages. I see every such dpkg|grep call
  takes about 0.3 seconds on lizard, i.e. 30% of the 1 second
  default delay between checks, which I suspect is enough to slow
  down the package installation/removal we're exercising.

• Update expected title of the GitLab page we use

• Rubocop: target Ruby 2.5 (Buster).