fix #15127 ActiveJob::DeserializationError thrown

Merged Minqi Pan requested to merge pmq20/gitlab-ce:issues/15127 into master

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 after_commit hook which is the right way to do it.