FEATURE: Implement new onboarding popups (#18362)

This commit introduces a new framework for building user tutorials as
popups using the Tippy JS library. Currently, the new framework is used
to replace the old notification spotlight and tips and show a new one
related to the topic timeline.

All popups follow the same structure and have a title, a description and
two buttons for either dismissing just the current tip or all of them
at once.

The state of all seen popups is stored in a user option. Updating
skip_new_user_tips will automatically update the list of seen popups
accordingly.

app/serializers/current_user_serializer.rb

@@ -68,6 +68,7 @@ class CurrentUserSerializer < BasicUserSerializer
              :timezone,
              :featured_topic,
              :skip_new_user_tips,
+             :seen_popups,
              :do_not_disturb_until,
              :has_topic_draft,
              :can_review,
@@ -282,6 +283,14 @@ class CurrentUserSerializer < BasicUserSerializer
     object.user_option.skip_new_user_tips
   end
 
+  def seen_popups
+    object.user_option.seen_popups
+  end
+
+  def include_seen_popups?
+    SiteSetting.enable_onboarding_popups
+  end
+
   def include_primary_group_id?
     object.primary_group_id.present?
   end