DEV: Make discourse-common an Ember addon. (#9578)

This is to help with the migration to Ember CLI. In the current running
version of Discourse everything should be the same as before, just with
a few extra files that are not used. However, using Ember CLI this can
be installed as an Ember addon.

Co-Authored-By: Jarek Radosz <jradosz@gmail.com>

app/assets/javascripts/discourse-common/addon/lib/attribute-hook.js

@@ -0,0 +1,42 @@
+// FROM: https://github.com/Matt-Esch/virtual-dom
+// License: MIT
+
+function AttributeHook(namespace, value) {
+  if (!(this instanceof AttributeHook)) {
+    return new AttributeHook(namespace, value);
+  }
+
+  this.namespace = namespace;
+  this.value = value;
+}
+
+AttributeHook.prototype.hook = function(node, prop, prev) {
+  if (
+    prev &&
+    prev.type === "AttributeHook" &&
+    prev.value === this.value &&
+    prev.namespace === this.namespace
+  ) {
+    return;
+  }
+
+  node.setAttributeNS(this.namespace, prop, this.value);
+};
+
+AttributeHook.prototype.unhook = function(node, prop, next) {
+  if (
+    next &&
+    next.type === "AttributeHook" &&
+    next.namespace === this.namespace
+  ) {
+    return;
+  }
+
+  var colonPosition = prop.indexOf(":");
+  var localName = colonPosition > -1 ? prop.substr(colonPosition + 1) : prop;
+  node.removeAttributeNS(this.namespace, localName);
+};
+
+AttributeHook.prototype.type = "AttributeHook";
+
+export default AttributeHook;