Add objectFit prop to Thumbnail component

[lyzadanger]

Right now a !-rule is required to adjust object-fit for content within a Thumbnail:

        <Thumbnail loading={isLoadingBook} ratio="4/5">
          {selectedBook && (
            <img
              alt={`Book cover for ${selectedBook.title}`}
              className="!object-contain"
              src={selectedBook.cover_image}
              data-testid="cover-image"
            />
          )}
        </Thumbnail>

AspectRatio, which is used by Thumbnail, takes an objectFit prop but Thumbnail doesn't take or set objectFit, so we're stuck with the default object-cover when using Thumbnail currently (well, this can be worked around temporarily with a !-rule).

Let's add flexibility by adding an objectFit prop to Thumbnail and forward that on to AspectRatio.