[v9_9] stats improvements (newstats only)

[v9_9] stats improvements (newstats only)

3699.  [bug]           Improvements to statistics channel XSL stylesheet:
                       the stylesheet can now be cached by the browser;
                       section headers are omitted from the stats display
                       when there is no data in those sections to be
                       displayed; counters are now right-justified for
                       easier readability. [RT #35117]