Create Debian 9 "Stretch" branch

[mdPlusPlus]

• h2 instead of spdy for nginx from debian repo
• MariaDB instead of MySQL (including a conversion script?)
• certbot in repo? backports required?
• switch to php7 (no stable suhosin)?

Resources to consider (performance):

• https://www.scalingphpbook.com/blog/2014/02/14/best-zend-opcache-settings.html (2014)
• https://haydenjames.io/php-fpm-tuning-using-pm-static-max-performance/ (2017)
• https://haydenjames.io/php-benchmarks-opcache-performance-tweaks/ (2017)
• https://haydenjames.io/nginx-tuning-tips-tls-ssl-https-ttfb-latency/ (2018)
• https://haydenjames.io/php-performance-opcache-control-panels/ (2017)
• https://ma.ttias.be/a-better-way-to-run-php-fpm/ (2014)

[mdPlusPlus]

Stretch should probably get skipped. Debian Buster is soon going to be frozen: https://wiki.debian.org/DebianBuster

[mdPlusPlus]

Went straight to Debian 11 "Bullseye", but didn't check any of the additional ressources.
Leaving this issue open to not forget about those.

[mdPlusPlus]

Should be Debian and Ubuntu version agnostic now. Additional resources still not considered.