Add new messages for article and image attribution

The new renderer will pull these messages from the wiki so that they
can be customized there.

Relies on:
* https://gerrit.wikimedia.org/r/#/c/145187/
* https://gerrit.wikimedia.org/r/#/c/145189/

Change-Id: Ifd80ca8c9b4f44100506fd52daa98093f88f0b5e
(cherry picked from commit 4f60b4ef561a855299f509d5a7d9f091e41853a8)