This is just a massive 's/Icedove/Thunderbird/' both in terms of
content and filenames. No logic is changed, which should ease
reviewing of this massive commit.

Will-fix: #12242

This will install thunderbird 1:45.8.0-3+tails2 built for amd64 on
Debian Stretch. Naturally it is a first step for the Icedove →
Thunderbird migration as well. And this package contains an AppArmor
profile (unlike the Debian Jessie package).

Refs: #12242
Will-fix: #11712, #11973

These packages also has the patch from the following upstream ticket
applied:

    https://bugzilla.mozilla.org/show_bug.cgi?id=1281959

We enable browser.download.forbid_open_with so the "Open with..."
option is hidden in the attachment download dialog, since
Thunderbird's AppArmor profile does not allow starting applications in
Tails.

Refs: #11964

Merge remote-tracking branch 'origin/feature/12409-improved-vagrant-base-box-versioning' into stable

Fix-committed: #12409

Merge remote-tracking branch 'origin/bugfix/12527-really-fix-unmounting-on-build-cleanup' into feature/stretch

Fix-committed: #12527

Merge remote-tracking branch 'origin/stable' into feature/12409-improved-vagrant-base-box-versioning

It seems fuser binary was missing in the build VM for the cleanup code
to correctly work and kill processes that prevent the unmounting of the
build directory.

Will-fix: #12527

There's some overhead with each ssh call that we might as well avoid,
and each scp invokation prints a warning ("Warning: Permanently added
[...] to the list of known hosts") that is nicer to just see once.

I added the arguments to these in anticipation of them being useful
for other boxes. That didn't turn out to be the case, so let's remove
them.

In `rm -f ... || :` the `|| :` part is not needed since `-f` ensures
that `rm` always will succeed even if some files given as arguments
are missing.

* Committing anything to vagrant/definitions/tails-builder means there
  is a new version of the builder VM (and it will automatically be
  generated next Tails build)
* Separately track the time-based snapshot serials used to generate
  the builder

These changes makes it extremely convenient to release new boxes (just
commit) and avoids issues where commits made to
vagrant/definitions/tails-builder may affect *some* users base box
generation, but not others, depending on when they Git fetch (see
ticket #12409, comment 32).

Will-fix: #12409

Refs: #11972

Refs: #11972

Refs: #11972

Required to use killall to kill remaining gpg-agents.

Refs: #11972

Otherwise umounting the tmp build dir fails.

Refs: #11972

 Conflicts:
	Rakefile
	auto/build
	auto/scripts/tails-custom-apt-sources
	vagrant/provision/setup-tails-builder