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

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

The issue names no files, tests, or entry points. Locate the flow that creates a user's supervisor participation, then inspect how e-mail delivery is handled and queued; done means the newly added supervisor receives a queued notification every time.

Written by the indexing model from the issue text.