DEV: Remove user options from current user serializer (#19089)

User options were serialized at the root level of CurrentUserSerializer,
but UserSerializer has a user_option field. This inconsistency caused
issues in the past because user_option fields had to be duplicated on
the frontend.

app/serializers/current_user_option_serializer.rb

@@ -0,0 +1,33 @@
+# frozen_string_literal: true
+
+class CurrentUserOptionSerializer < ApplicationSerializer
+  attributes :mailing_list_mode,
+             :external_links_in_new_tab,
+             :enable_quoting,
+             :dynamic_favicon,
+             :automatically_unpin_topics,
+             :likes_notifications_disabled,
+             :hide_profile_and_presence,
+             :title_count_mode,
+             :enable_defer,
+             :timezone,
+             :skip_new_user_tips,
+             :default_calendar,
+             :bookmark_auto_delete_preference,
+             :seen_popups,
+             :should_be_redirected_to_top,
+             :redirected_to_top,
+             :treat_as_new_topic_start_date,
+
+  def likes_notifications_disabled
+    object.likes_notifications_disabled?
+  end
+
+  def include_redirected_to_top?
+    object.redirected_to_top.present?
+  end
+
+  def include_seen_popups?
+    SiteSetting.enable_user_tips
+  end
+end