Removed or not synced because of errors: ar, fa, hu, kk.

Custom APT repo: document how to add an APT overlay and find out how the overlay suite for a given topic branch is called.

Custom APT repo: move relevant info to the place where one is looking when they want to upload a package.

The Overview section explains the branch → APT suite mapping, but it's easy to
miss when one is focused on the "Importing a new package" howto, which can
waste developer time: if they don't push a branch before uploading,
their upload will be rejected.

In practice, there's rarely any reason to follow the doc we have for "Creating
a new branch" except before uploading a new package. So let's move these
instructions to where they're needed, i.e. "Importing a new package".

The update #6156 patchset renamed
mailnews.auto_config.account_constraints.ssl_only pref to
mailnews.auto_config.ssl_only_mail_servers. Let's set both.

Also, remove obsolete comment claiming that this is the default:
as of thunderbird 60.5.1-1~deb9u1.0tails1,
mailnews.auto_config.ssl_only_mail_servers is set to false by default.

Headers are now gathered as a list instead of a table row, so fetching
all table rows without recursion returns only messages; drop the shift
accordingly.

The context of the third hunk changed:

    -   @{thunderbird_executable} ixr,
    +   /usr/lib/thunderbird/thunderbird{,-bin} ixr,

Merge remote-tracking branch 'origin/bugfix/16566-asp-dl-failure-case-C' into stable (Fix-committed: #16566)

We had some specific check regarding the XPI namespace of Thunderbird's
language packs, to make sure our specific code was working fine. This
was removed when moving to a newer version of Thunderbird, so let's get
rid of that check.

It's the first snapshot that has Linux 4.19.28-2, which fixes
the https://bugs.debian.org/924545 regression.

We want to run the upgrade even if the install step fails, so let's ignore its exit code.

Will-fix: #15957