Include thumbnails by default in post selector

[mboynes]

Images can add a lot of polish to a product. The post selector modal would look even better if the grid of posts included thumbnails by default. This would also better align with the sidebar representation, which does include the thumbnail.

[mogmarsh]

It looks like if we always add the _embed param, we should get _embedded.wp:featuredmedia and then hopefully we can figure out a good size to use.

[ellm]

I submitted #220 to get this into place.