mason/dhandler, static/tgal.css: Embrace idea of multiple thumbnail sets.

Rather than a single row of thumbnails, the code is now structured so
that it might have several at the same time, with different sizes.  CSS
media queries can control which one actually gets displayed.

Internally, we make two passes over a directory, fetching the images the
first time through, and making scaled thumbnails afterwards, and
introduce loops over the available sizes (though currently there's only
the one).

We're actually just making the same display as before, so there's still
no functional change.