Fix WebPush (regression from #4524) (#4778)

* Fix NoMethodError in Web::PushSubscription

```
undefined method `site_contact_email' for #<Class:0x00005976d13c40>

/usr/local/bundle/gems/activerecord-5.1.3/lib/active_record/dynamic_matchers.rb:22:in `method_missing'
/usr/local/bundle/gems/attr_encrypted-3.0.3/lib/attr_encrypted.rb:295:in `method_missing'
/usr/local/bundle/gems/attr_encrypted-3.0.3/lib/attr_encrypted/adapters/active_record.rb:129:in `method_missing_with_attr_encrypted'
/mastodon/app/models/web/push_subscription.rb:53:in `push_payload'
```

* Specify serializer in Web::NotificationSerializer