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/controllers/users_controller.rb

@@ -1966,7 +1966,7 @@ class UsersController < ApplicationController
     end
 
     result = params
-      .permit(permitted, theme_ids: [])
+      .permit(permitted, theme_ids: [], seen_popups: [])
       .reverse_merge(
         ip_address: request.remote_ip,
         registration_ip_address: request.remote_ip