FIX: do not insert conflict rows into category user

This could fail under certain race conditions

db/post_migrate/20191107190330_remove_suppress_from_latest_from_category.rb

@@ -15,7 +15,8 @@ class RemoveSuppressFromLatestFromCategory < ActiveRecord::Migration[6.0]
 
       # We shouldn't encourage to have more than 10 categories in `default_categories_muted` site setting.
       if ids.count <= 10
-        DB.exec(<<~SQL, muted: CategoryUser.notification_levels[:muted])
+        # CategoryUser.notification_levels[:muted] is 0, avoid reaching to object model
+        DB.exec(<<~SQL, muted: 0)
           INSERT INTO category_users (category_id, user_id, notification_level)
             SELECT c.id category_id, u.id user_id, :muted
             FROM users u
@@ -25,6 +26,7 @@ class RemoveSuppressFromLatestFromCategory < ActiveRecord::Migration[6.0]
                   AND c.id = cu.category_id
             WHERE c.suppress_from_latest = TRUE
               AND cu.notification_level IS NULL
+          ON CONFLICT DO NOTHING
         SQL
 
         DB.exec(<<~SQL, value: ids.join("|"))