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
Merge "Add new messages for article and image attribution"