Import compressed mbox file

[Vigilans]

Currently IETNG does not do compressed import, see #390.

Linux kernel mailing lists in web supports downloading the thread in compressed mbox (mbox.gz) format, e.g. https://lore.kernel.org/lkml/20230823115048.823011-4-masahiroy@kernel.org/ with mbox.gz download link https://lore.kernel.org/lkml/20230823115048.823011-4-masahiroy@kernel.org/t.mbox.gz.

If IETNG supports import from mbox.gz, then when importing we can directly fill the download link to the dialog, let it download the mbox and automatically import to Thunderbird, instead of having to download manually to local, uncompress it and import with IETNG.