Add antiproxying protection.

Try to prevent malicious sites like danbooru.me or idanbooru.com from
proxying our site and inserting ads. If we detect that we're not running
on the real site, then we redirect to the real site.

app/javascript/src/javascripts/common.js.erb

@@ -19,6 +19,14 @@ $(function() {
     $('#notice').fadeOut("fast");
     e.preventDefault();
   });
+
+  const CANONICAL_DOMAIN = <%= Danbooru.config.domain.to_json.html_safe %>;
+  const CANONICAL_HOSTNAME = <%= Danbooru.config.hostname.to_json.html_safe %>;
+  const ENABLE_ANTIPROXYING = <%= Danbooru.config.enable_antiproxying?.to_json.html_safe %>;
+
+  if (ENABLE_ANTIPROXYING && !location.hostname.endsWith(CANONICAL_DOMAIN)) {
+    location.hostname = CANONICAL_HOSTNAME;
+  }
 });
 
 window.submitInvisibleRecaptchaForm = function () {