Implement D8 flood control

contact_flood_control() in ContactFormBlock::build() appears to be totally bogus - I can find no D6, D7 or D8 API reference for this? An attempt is made to include contact.pages.inc, but (in D7) that contains no such function anyway.

I guess we should be retrieving contact module's flood configuration, then doing doing something like:

$flood = \Drupal::flood();
if (!$flood->isAllowed('contact', $time, $interval) {
  // Set a message and throw a Symfony access denied error?
}

But I don't think flood control on the core contact page is working yet anyway.