UX: Various improvements to welcome topic CTA (#21010)

- Update welcome topic copy
- Edit the welcome topic automatically when the title or description changes
- Remove “Create your Welcome Topic” banner/CTA
- Add "edit welcome topic" user tip

app/models/user.rb

@@ -355,6 +355,7 @@ class User < ActiveRecord::Base
         post_menu: 3,
         topic_notification_levels: 4,
         suggested_topics: 5,
+        welcome_topic: 6,
       )
   end