fix #15127 ActiveJob::DeserializationError thrown
send_devise_notification pre-maturely enqueued the task when the user instance
has not yet been committed into the database, causing a record-not-found in the
other sidekiq process.
devise-async has already been taking care of asynchronous mail sending, we just
need to run it inside queue
mailers instead of
mailer to enable it.
The implementation of
devise-async enqueues the task in
which is the right way to do it.