frontend: make domain optional

Making the domain optional allows usage of bare hostnames for
the servers (e.g. when they're statically configured) and even
IP addresses if someone is so inclined (although presentation
might suffer in this case).

frontend/main.go

@@ -80,8 +80,6 @@ func main() {
 
 	if *serversPtr == "" {
 		panic("no server set")
-	} else if *domainPtr == "" {
-		panic("no base domain set")
 	}
 
 	setting = settingType{