FEATURE: Send notifications to admins when new features are released (#19460)

This commit adds a new notification that gets sent to admins when the site gets new features after an upgrade/deploy. Clicking on the notification takes the admin to the admin dashboard at `/admin` where they can see the new features under the "New Features" section.

Internal topic: t/87166.

app/models/notification.rb

@@ -139,6 +139,7 @@ class Notification < ActiveRecord::Base
                         assigned: 34,
                         question_answer_user_commented: 35, # Used by https://github.com/discourse/discourse-question-answer
                         watching_category_or_tag: 36,
+                        new_features: 37,
                        )
   end