restbase: Use thumbnail when generating thumbnail

Rather than manipulating the URL of the original image to get the URL of
the appropriately sized thumbnail, manipulate the URL of the thumbnail
image.

While we could manipulate either the thumbnail or original image URL,
there are subtle differences between the two, so manipulating the latter
makes the generateThumbnailData function as simple as possible, e.g. we
don't have to splice in "/thumb" after "/commons".

Also, ensure that the thumbnail's dimensions are scaled appropriately.

Bug: T156800
Change-Id: I6825bad14b1131dc81f23dcf120cf8ffb7d7b4f6