Merge branch 'headercheck-localhost-ipv6' into '2.x'

Also block local access using IPv6

See merge request !63

src/main/java/de/thm/arsnova/controller/WelcomeController.java

@@ -85,7 +85,7 @@ public class WelcomeController extends AbstractController {
 			final HttpServletRequest request
 		) {
 		/* Block requests from the server itself to prevent DoS attacks caused by request loops */
-		if ("127.0.0.1".equals(request.getRemoteAddr())) {
+		if ("127.0.0.1".equals(request.getRemoteAddr()) || "::1".equals(request.getRemoteAddr())) {
 			throw new BadRequestException("Access to localhost not allowed.");
 		}
 		/* Block requests to servers in private networks */