Add /measure route

This provides a method to measure the size of a directory (defined as
the sum of the sizes of all the files it contains and (recursively) the
sizes of all subdirectories.
Add directory size computation to web interface

It needs much more polish, but the prototype is basically functional
now.
Improve web display format and colors

Nothing really substantive here.  The code has been refactored a bit in
order to reuse the file-size formatting in the client-side JavaScript.