Fallback for 404ing blog post thumbnails

[rebeccacremona]

In this design, the list of blog posts is a series of labeled images. The logic is supposed to be as follows:

• if a post defines a special thumbnail, use that
• if not, and if a post contains one or more images, use the first image in the post
• if not, use one of the fallback images

This PR fixes that: as is, the special thumbnail is overridden by a first image.

And, it adds special handling for when the first image 404s using this strategy, which I hadn't seen before.

I note you have to do this in two spots: the HTML include used by Jekyll.... and the javascript file used to render search results.

Before:

After: