Widget: load secure images
As reported in reddit/reddit#1291, we've been loading some images in our embed
widgets (the old ones, not the new comment embeds) over http. This causes
warnings in most browsers when the embedding page is loaded over https, since
we dropping down to insecure elements.
Now we're always loading them over https. Alternatively, we could use
protocol-relative urls, but I figure there's no harm in always using https, and
it's simpler and causes fewer weird issues with browsers.