Fix Dana's comments, one of which exposed a bug: we weren't actually
fixing the diff. Because resolveImage can return multiple formats, and
only returns a self_link if a self_link is passed in, it was diffing
more often than not against a self_link supplied in the config. We just
had a bug in our CustomizeDiff function that concealed this.
I added the resolvedImageSelfLink helper function to turn the output
from resolveImage into a self_link, which fixes the problem. It also has
the knock-on effect of fixing #2067 at the same time, which is nice. I
decided to add another function instead of just modifying resolveImage
to always return a self_link because I don't want to deal with the
backwards compatibility problems of changing how we're storing a bunch
of things in state this close to 1.19.0. And honestly, we should
probably just be storing the self_link always, _anyways_.