(into master) Bug 992327 - Change directoryLinks and NewTabUtils to imageURI instead of imageURISpec

from bug: https://bugzilla.mozilla.org/show_bug.cgi?id=992327

from adw: hey, i saw the json in one of the bugs, and i feel bad about asking you to call the image uri property imageURISpec.  please feel free to change it back to imageURI if it's not too much trouble, preemptive r+!