exim: replace SQLite userdb lookup with static file

Instead of setting up keys, crontabs etc. to copy of a large SQLite file
from the IMAP server to the mail router that allows lookups for
arbitrary email addresses, use a static lookup against a local part list
and a fixed domain of wikimedia.org.

The local part list contains exactly 5 users and is the set of users
that have logged in to IMAP the past 3 months. This brings down the list
down from the 86 addresses that match against SQLite right now that are
remnants of Wikimedia's distant past.

Change-Id: Ie645a026d45b922d14dc1c3124f4eac846342e03