Migrate old web push subscriptions to ensure deliveries (#7764)

db/migrate/20180609104432_migrate_web_push_subscriptions2.rb

@@ -0,0 +1,17 @@
+class MigrateWebPushSubscriptions2 < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def up
+    Web::PushSubscription.where(user_id: nil).select(:id).includes(:session_activation).find_each do |subscription|
+      if subscription.session_activation.nil?
+        subscription.delete
+      else
+        subscription.update_attribute(:user_id, subscription.session_activation.user_id)
+      end
+    end
+  end
+
+  def down
+    # Nothing to do
+  end
+end