Add 'motd' module from MediaWiki-Vagrant

I noticed Coren doing some motd stuff in Ib75c18f75, which reminded me that we
have a handy module for this in MediaWiki-Vagrant that has been working well.

Change-Id: I2a8708c4f7940bb8c1e7c6f9d5d330cc8279fa21
Merge "Add 'motd' module from MediaWiki-Vagrant" into production