.htaccess file: Add rules to forward old URLs to new pages

The .htaccess file (in the foomatic-web-app repo)  allows to carry any amount of rules to redirect old URLs (which could be linked from somewhere)  to pages of the new site. Feel free to add rules here.

The most important rule, letting www.openprinting.org be redirected to https://openprinting.github.io, I have already added.