Send an email when a process is completed

Refs https://gitlab.com/teklia/requests/-/issues/867

When a process is completed (whatever its filnal state), a call to UpdateTaskFromAgent is made by the ponos agent. Repository process should never send any emails.

When a final state is reached, a mail should be sent to the process creator with:

• the process ID
• its overall final state
• the state of all tasks
• a link to view the process details & logs

We could make some followups with worker activity stats, but it should be sufficient for now.

The endpoint should simply trigger an async task dedicated to building and sending the actual email when a process is complete.