Make it possible to customize a section on the homepage and in the footer

It has been suggested that it would be nice if we could customize the homepage of debusine.debian.net to point to https://wiki.debian.org/DebusineDebianNet. In general, it's a fair request that a debusine administrator ought to be able to add a small blurb explaining the purpose of the service, who can use it and what for, etc.

One simple way to achieve this is to integrate a template snippet that can be overridden by the local administrators by putting it in /var/lib/debusine/server/templates/. It's nice to also have something that makes it easy to extend/customize the footer in a similar way.