FEATURE: Unseen feature indicator in admin sidebar (#28397)

This commit adds a blue dot next to the "What's New"
link in the admin sidebar if the user has not seen the
new features yet, as a followup to 3e5976f843
which removed the tab on the dashboard that had this same
functionality.

When the admin visits the "What's New" page they count
as having seen all the features straight away. This could
be something we want to change, but for now this keeps the
same functionality.

app/serializers/current_user_serializer.rb

@@ -77,7 +77,8 @@ class CurrentUserSerializer < BasicUserSerializer
              :can_view_raw_email,
              :use_glimmer_topic_list?,
              :login_method,
-             :render_experimental_about_page
+             :render_experimental_about_page,
+             :has_unseen_features
 
   delegate :user_stat, to: :object, private: true
   delegate :any_posts, :draft_count, :pending_posts_count, :read_faq?, to: :user_stat
@@ -142,6 +143,14 @@ class CurrentUserSerializer < BasicUserSerializer
     object.staff?
   end
 
+  def has_unseen_features
+    DiscourseUpdates.has_unseen_features?(object.id)
+  end
+
+  def include_has_unseen_features?
+    object.staff?
+  end
+
   def render_experimental_about_page
     object.in_any_groups?(SiteSetting.experimental_redesigned_about_page_groups_map)
   end