fix: convert mailman links to lists.fsfe.org #1425

Converts all previous mail.fsfeurope.org and mailman.fsfeurope.org links to the new https://lists.fsfe.org domain. This actually solves a lot of broken links.

Note that this changes both links and text.

Executed commands:

find . -iname '*.xml'  -exec perl -pi -e 's/http(s)?:\/\/mail(man)?\.fsfe(urope)?\.org\//https:\/\/lists\.fsfe\.org\//g' {} \;
find . -iname '*.xhtml'  -exec perl -pi -e 's/http(s)?:\/\/mail(man)?\.fsfe(urope)?\.org\//https:\/\/lists\.fsfe\.org\//g' {} \;

I've tested some links to verify that the replacement worked, but not all.

Ah, very nice, thanks!