Fix Card Opened Webhook can not be disabled.

Thanks to mvanvoorden and xet7 !

Closes #2932,
closes #2889,
closes #2666

server/card-opened-webhook.js

@@ -1,5 +1,7 @@
 Meteor.startup(() => {
-  if (process.env.CARD_OPENED_WEBHOOK_ENABLED) {
+  if (process.env.CARD_OPENED_WEBHOOK_ENABLED === 'true') {
     Meteor.settings.public.CARD_OPENED_WEBHOOK_ENABLED = true;
+  } else {
+    Meteor.settings.public.CARD_OPENED_WEBHOOK_ENABLED = false;
   }
 });