Send e-mail to user when their participation as examiner is created

Every time a user is added as an examiner, such user should be notified by e-mail. The sending of the e-mail must be queued.

Start by locating the code path where a user is added as an examiner. Review the project's existing e-mail and queue handling before defining the notification behavior. The work is done when each newly added examiner receives an e-mail through the queue.

Written by the indexing model from the issue text.