UX: Text customization for different languages. (#11729)

Admins can now edit translations in different languages without having to change their locale. We display a warning when there's a fallback language set.

app/assets/javascripts/admin/addon/models/site-text.js

@@ -1,10 +1,10 @@
 import RestModel from "discourse/models/rest";
 import { ajax } from "discourse/lib/ajax";
-const { getProperties } = Ember;
+import { getProperties } from "@ember/object";
 
 export default RestModel.extend({
-  revert() {
-    return ajax(`/admin/customize/site_texts/${this.id}`, {
+  revert(locale) {
+    return ajax(`/admin/customize/site_texts/${this.id}?locale=${locale}`, {
       type: "DELETE",
     }).then((result) => getProperties(result.site_text, "value", "can_revert"));
   },