FIX: select-kit was in the wrong place

`app/` means we want to merge it into our application there. `addon`
means give it its own module namespace, which is what we wanted.

app/assets/javascripts/select-kit/addon/components/topic-notifications-button.js

@@ -0,0 +1,28 @@
+import Component from "@ember/component";
+import { action, computed } from "@ember/object";
+
+export default Component.extend({
+  layoutName: "select-kit/templates/components/topic-notifications-button",
+  classNames: ["topic-notifications-button"],
+  classNameBindings: ["isLoading"],
+  appendReason: true,
+  showFullTitle: true,
+  placement: "bottom-start",
+  notificationLevel: null,
+  topic: null,
+  showCaret: true,
+  isLoading: false,
+  icon: computed("isLoading", function() {
+    return this.isLoading ? "spinner" : null;
+  }),
+
+  @action
+  changeTopicNotificationLevel(levelId) {
+    if (levelId !== this.notificationLevel) {
+      this.set("isLoading", true);
+      this.topic.details
+        .updateNotifications(levelId)
+        .finally(() => this.set("isLoading", false));
+    }
+  }
+});