FEATURE: English locale with international date formats

Makes en_US the new default locale

test/javascripts/components/d-editor-test.js.es6

@@ -646,7 +646,7 @@ componentTest("clicking the toggle-direction button toggles the direction", {
   template: "{{d-editor value=value}}",
   beforeEach() {
     this.siteSettings.support_mixed_text_direction = true;
-    this.siteSettings.default_locale = "en";
+    this.siteSettings.default_locale = "en_US";
   },
 
   async test(assert) {

test/javascripts/fixtures/about.js.es6

@@ -20,7 +20,7 @@ export default {
       description:
         "Discussion about the next-generation open source Discourse forum software",
       title: "Discourse Meta",
-      locale: "en",
+      locale: "en_US",
       version: "2.2.0.beta8",
       https: true,
       admins: [

test/javascripts/test_helper.js

@@ -16,7 +16,7 @@
 //= require preload-store
 
 //= require locales/i18n
-//= require locales/en
+//= require locales/en_US
 
 // Stuff we need to load first
 //= require vendor