Make missing.php aware of interwiki prefixes

At the moment, if you are on a wiki belonging to a minor project/small
language, where not all sister projects may exist, double interwiki links
can sometimes fail. (e.g. go to uk.wikivoyage.org,  link to :ko:v: and
:v:ko: and you will get a broken page on Incubator.)

This fixes missing.php to make those double interwikis work, by adding a
lookup to interwiki.cdb if the page title contains ":".

NOTE: I tested this using Cdb_PHP, but couldn't test with dba, since
that extension doesn't seem to exist (?) on Windows. If someone could
just run this on a test server or something to make sure the dba_ calls
work, that would be appreciated!

Bug: 48035
Bug: 7496
Change-Id: Ie398ebd1b77cbcaad01355f4147627495b53ca91