CardDAV integration in Roundcube on Debian

After much fiddling around, I finally worked out how to get the CardDAV plugin to work on my Debian box. The main problem was working out where to place the plugin's source code and configuration files, and where to create symlinks so that the Debian-packaged version of Roundcube finds the plugin and picks up its config file. Here is the full account of how to install and configure Roundcube and the CardDAV plugin on a Debian box that tracks the "testing" distribution.