Expose iw_trans in siteinfo API interwikimap prop

It is already exposed by the Interwiki extension, deployed on all
Wikimedia wikis. I don't see why we would not expose it here.

Change-Id: I61712bac7a2ab5672a79de866617af2915b9e41b
Merge "Expose iw_trans in siteinfo API interwikimap prop"