Lightbox doesn't show embedded Dropbox images

[gnprice]

This is an issue whose symptoms look a lot like #2654, but the cause is a very close cousin of #2895. It's a bit of unfinished business that #2899 didn't yet fix.

To reproduce:

• Paste into a message a Dropbox shared link that happens to be to (something with the filename extension of) an image, say .png.
• In the mobile app, touch the image.
• We show a blank lightbox.

What's happening is that the HTML for one of these looks like this (edited slightly for readability):

<div class="message_inline_image">
  <a href="https://www.dropbox.com/s/b7ke6alg3n2/foo.png?dl=0" target="_blank" title="foo.png">
    <img src="https://www.dropbox.com/s/b7ke6alg3n2/foo.png?dl=1">
  </a>
</div>

Note the two different URLs -- the img refers to the actual image, and the link refers to the Dropbox shared-link page around it. The lightbox should of course use the image.

Instead, we try to use the link as if it were an image, and of course that fails.