Embed SVG icons directly into output.

Inlining SVGs directly in the markup seems to interoperate somewhat
better than linking SVGs in img elements, and we can fairly easily
create fallback markup that degrades to a raster image.

Currently this is only applicable to the file listings because the
fallback markup is hardcoded to the icons on these pages.  This will
need to be expanded in the future.

The results seem pretty good, although nothing I do in old Mozilla
versions seems to be able to get the stylesheet sizes to properly
apply to SVG elements, so the icons are rendered correctly but at
the wrong scale.  SVG support can be disabled in this browser and
the fallback works properly.  More work may be required.