DEV: Add top level `<div>` when using Ember CLI (#12878)

Eventually we'd like to remove this but it makes our Ember CLI build
consistent with the non-ember version.

app/assets/javascripts/discourse/app/components/discourse-root.js

@@ -0,0 +1,10 @@
+import Component from "@ember/component";
+
+let componentArgs = { tagName: "div" };
+
+// TODO: Once we've moved to Ember CLI completely we can remove this.
+if (!Ember.FEATURES.EMBER_GLIMMER_SET_COMPONENT_TEMPLATE) {
+  componentArgs = { tagName: "" };
+}
+
+export default Component.extend(componentArgs);

app/assets/javascripts/discourse/app/templates/application.hbs

@@ -1,3 +1,4 @@
+{{#discourse-root}}
   {{d-document}}
   {{plugin-outlet name="above-site-header" tagName=""}}
   {{site-header canSignUp=canSignUp
@@ -40,3 +41,4 @@
   {{#if showFooterNav}}
     {{footer-nav}}
   {{/if}}
+{{/discourse-root}}