[RichString] Add markdown support Supporting markdown requires the 'Markdown' python packages, and since it widely available on the target platforms adding it as a strong requirements will not be a big constraint. Closes #3814302

etc/init.d
etc/cubicweb.d
etc/bash_completion.d
usr/bin
usr/share/doc/cubicweb-ctl
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances