better protection for smart_host secret #142
Description
smart host configuration for email forwarding is done through an environment variable, which leads to potentially exposed secrets when the SMTP relay is password-protected.
Storing this in the database as a configuration key would make it somewhat less problematic, at the cost of making the secret readable by any instance admin. Furthermore, it would be stored as clear text in the database, making it very vulnerable to exploit in case of a data leak.
Client certificates for database connections pose a similar problem that is solved by providing a mounted file from which the secret is read. Even though SMTP relays generally won't support client certificates, we could use a similar technique to store a clear-text password.