I'm not sure that changing the URLs is a good idea.  I'd rather use
.htaccess and mod_rewrite to redirect short URLs to their permanent
long equivalents.  Nobody else seems to mind though, so I've merged
Gianluca's solution with a few changes:
  * Since we're truncating bug IDs, truncate comment IDs too.
  * Use libbe.util.id._truncate to generate the short IDs, so that `be
    html` truncation is consistent with general BE truncation.
  * Updated cross-linking code to match.

