Smaller thumbnails for lower-density displays

[DangerOnTheRanger]

The thumbnails that Maniwani generates are currently quite large (made to fit in a 500x500-pixel box), and while this works well in some cases, oftentimes on smaller or lower-density displays they can be a waste. It would be good for Maniwani to generate smaller thumbnails for those kinds of clients. For picking the correct thumbnail resolution, the <source> tag can be used in conjunction with the srcset attribute.

Despite storing a few additional thumbnail images per attachment, say, a 300x300 one and a 150x150 one, hosting costs for Maniwani installations should fall by a little bit with this change. Storage is cheaper than bandwidth, after all. Clients should also benefit from somewhat faster load times.