Use an `after_commit` instead of an `after_save`. The `after_save` could
cause intermittent problems due to Sidekiq starting before the
record/transaction actually gets committed to the database. In those
cases, the record wouldn't be found and cause an error.
With an `after_commit` callback, the record is sure to be committed
before Sidekiq takes over.
Additionally, limit the callback to `:create` and `:update` because
otherwise it will be active on `:destroy` by default also. This work
isn't relevant on destroy, so don't do it then.
Update the tests:
* Update labels
* Use `build` instead of `create` because we're saving the record at the
end anyway so we don't need an object that starts out persisted
* Use `truncation: true` for these tests because otherwise the `commit`
callback doesn't get called due to transactional teardown
Refs #6407