Make BounceHandler extension work on test2wiki

Before mass-rollout in production, its safe to do a prod level test in test2wiki
* Removed +local_domain from the realm switch as it was no longer required there
* Made exim listen to bounces with $local_part as 'wikimedia.org'

Change-Id: I7badb2383a3e4d6d78e28bea6a97c5a51d5be64d
Merge "Make BounceHandler extension work on test2wiki" into production