Bug 990977 - Sponsored tiles: don't expose explanation URL as a localizable string [r=adw]
Hardcode the url into xul instead of dtd.
Bug 992327 - Change directoryLinks and NewTabUtils to imageURI instead of imageURISpec [r=adw]
Replace reference data and uses of s/Spec//.
Bug 993581 - use an empty directory tiles data source pref [r=adw]
Hardcode empty object for now.