Split mail smarthost for Labs
Split role::mail::sender's config per realm and use a separate one for
Labs that a) rewrites to firstname.lastname@example.org, not email@example.com, b)
blackholes mail destined for root, rather than sending it to production
roots, c) drops the special-MediaWiki handling of emails.
On a later step, Labs could get its own mail relays entirely; this split
config would allow for this.
For mails in Labs to properly work after this change, wmflabs.org should
become a mail domain (MX records + a simple alias file defining at least
the root alias).
This will significantly reduce the amount of cronspam production roots