api: add option to forbid new owner registrations

Closes https://gitlab.com/commento/commento-ce/issues/10

api/owner_new.go

@@ -3,6 +3,7 @@ package main
 import (
 	"golang.org/x/crypto/bcrypt"
 	"net/http"
+	"os"
 	"time"
 )
 
@@ -11,6 +12,10 @@ func ownerNew(email string, name string, password string) (string, error) {
 		return "", errorMissingField
 	}
 
+	if os.Getenv("FORBID_NEW_OWNERS") == "true" {
+		return "", errorNewOwnerForbidden
+	}
+
 	ownerHex, err := randomHex(32)
 	if err != nil {
 		logger.Errorf("cannot generate ownerHex: %v", err)