Consider publishing a tarball with all Tor Browser langpacks

At the moment, during a build of Tails, we download all Tor Browser Linux x86_64 tarballs, in order to grab every langpack XPI, which we then all ship in our images. This causes trouble for 2 categories of Tails contributors:

• The Tails release manager has to semi-automatically download all these tarballs and then upload them to the Tails infrastructure (that's because at this time, the tarballs are available only on people.torproject.org, and later they will disappear from there; we need a stable URL that we can encode in our Git tree, for convenience and reproducibility purposes); this consumes time, bandwidth, and storage.
• After a new Tor Browser has been imported into Tails, next time any Tails developer wants to build Tails, as part of the build, downloads each of these tarballs. This consumes time, bandwidth, and patience.

One possible solution for these problems would be to publish, on Tor Browser team's side, an additional tarball, that contains all langpack XPIs, and nothing else. According to boklm, this tarball would about 16 MB large.

Then, the Tails release manager would only have to import the en-US tarball and this all-langpacks tarball. Similarly, Tails developers would also have to download only these 2 files.

This is not a duplicate of legacy/trac#17400 (moved) nor legacy/trac#12967 (moved), which are about providing end-users a multi-lingual Tor Browser.