summary: compute lang_stats consistently

Visiting a /statistics with py3 would fail with:
... in statistics
sorted(lang_stats, reverse=True, key=lambda k: k[1])[:10]
TypeError: '<' not supported between instances of 'dict' and 'dict'

The "summary" computation didn't have that problem. And it put '?' as
description for unknown extensions. And it had stable output as it also sorted
on the file extension as secondary key. Just use that.