LABS: set zerobanner to use lab's zerowiki

* Switched URL to http://zero.wikimedia.beta.wmflabs.org/w/api.php
* Added the code needed to switch from ZRMA to ZeroPortal on zerowiki
* The actual switch is still off, pending extension patches

Change-Id: I12ef65bdb3e119bac566f792e7d2019661c3b906