Disable ZeroPortal ext on metawiki

ZeroPortal is no longer hosted on meta, and needs to be deleted.

One possible issue: there were a number of Zero: namespace pages
that used a custom content handler. All these pages have already
been deleted, but there have been some issues with the core
unable to handle missing content models.

Change-Id: I4ad5f91689bce66e33778b34346db98a7e1a6486