radicale: enable & start service after install

Hi, on a fresh buster install, installed freedombox, then radicale according to the wiki (verified in a fresh VM too). Unfortunately, it was not running at /radicale/ for the web ui, or for DAVx5. According to radicale.NEWS, the service is disabled by default, so I had to enable it on the console and reboot. Please do this as part of the plinth install step, or at least have it hooked into "Enable application".