Embrace the idea of multiple thumbnail sizes.

This is all prep work for a forthcoming change.

  * Rename the `thumb' sized to be `bigthumb', contemplating other
    thumbnail sizes.

  * Pass a `$size' argument to the `.thumbnail' module.

  * Attach the thumbnail size name as an additional class to the
    `figure' and `figcaption' classes.  (We'll add it to `thumbstrip'
    and `gallery' too, later.)