FEATURE: Automatic translation and localization of posts, topics, categories (#1376)

Related: https://github.com/discourse/discourse-translator/pull/310

This commit includes all the jobs and event hooks to localize posts, topics, and categories.

A few notes:
- `feature_name: "translation"` because the site setting is `ai-translation` and module is `Translation`
- we will switch to proper ai-feature in the near future, and can consider using the persona_user as `localization.localizer_user_id`
- keeping things flat within the module for now as we will be moving to ai-feature soon and have to rearrange
- Settings renamed/introduced are:
  - ai_translation_backfill_rate (0)
  - ai_translation_backfill_limit_to_public_content (true)
  - ai_translation_backfill_max_age_days (5)
  - ai_translation_verbose_logs (false)

assets/javascripts/initializers/translation.js

@@ -0,0 +1,24 @@
+import { apiInitializer } from "discourse/lib/api";
+import cookie from "discourse/lib/cookie";
+
+export default apiInitializer((api) => {
+  const settings = api.container.lookup("service:site-settings");
+
+  if (!settings.discourse_ai_enabled || !settings.ai_translation_enabled) {
+    return;
+  }
+
+  api.registerCustomPostMessageCallback(
+    "localized",
+    (topicController, data) => {
+      if (!cookie("content-localization-show-original")) {
+        const postStream = topicController.get("model.postStream");
+        postStream.triggerChangedPost(data.id, data.updated_at).then(() => {
+          topicController.appEvents.trigger("post-stream:refresh", {
+            id: data.id,
+          });
+        });
+      }
+    }
+  );
+});