[5.6] Ability to set mail message ID right hand side using a config value

[themsaid]

This PR adds the ability to set the right hand side of the Message-id header to a specific domain.

To do this, you need to set the mail.domain config value to a valid domain.

[sisve]

No tests?

[taylorotwell]

We can probably move this into the singleton registration for the mailer, so we don't load this until it's needed?

[themsaid]

@taylorotwell moved it into the singleton and also added a condition to only run it a config value was set.

[tillkruss]

@themsaid: This should probably be either mentioned in the docs, or maybe even be added to config/mail.php.