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/multi-select/multi-select-header.js

@@ -0,0 +1,25 @@
+import SelectKitHeaderComponent from "select-kit/components/select-kit/select-kit-header";
+import { computed } from "@ember/object";
+import { makeArray } from "discourse-common/lib/helpers";
+
+export default SelectKitHeaderComponent.extend({
+  classNames: ["multi-select-header"],
+  layoutName:
+    "select-kit/templates/components/multi-select/multi-select-header",
+
+  selectedNames: computed("selectedContent", function() {
+    return makeArray(this.selectedContent).map(c => this.getName(c));
+  }),
+
+  selectedValue: computed("selectedContent", function() {
+    return makeArray(this.selectedContent)
+      .map(c => {
+        if (this.getName(c) !== this.getName(this.selectKit.noneItem)) {
+          return this.getValue(c);
+        }
+
+        return null;
+      })
+      .filter(Boolean);
+  })
+});