Translate statcmp.py to JavaScript

[fkaelberer]

This PR translates statcmp.py to JavaScript (compare #4496).

The translation is close to 1:1, with the following minor exceptions:

• Differences are displayed with opposite signs. -90ms and -90% now means that current is faster than baseline. Since all numbers are in ms, I thought that this shows lower is better more clearly. It also makes more obvious that it means 10x faster (compared to 1.9x faster).
• If results are not grouped by 'stat', then only Overall results are used (page request and rendering are dropped). I thought it is more intuitive to show the Overall score in this case, rather than the average of Overall, rendering, and page request (which is 2/3 of Overall). As a side effect, the significance test delivers different results in this case.
• The number of samples in each group is displayed (column Count).
• The javascript ttest delivers slightly different results python's ttest, although differences are not big.

[timvandermeij]

Nice work, thank you!