v1.21.0

Bump to 1.21.0: add [turf_stats] shortcode, fix corrupted nl_NL .mo

- New [turf_stats] shortcode: at-a-glance table (online now, today,
  yesterday, last 7 days, last 28 days) for a sidebar widget/Shortcode
  block, with metric/layout/items attributes.
- Fixed a corrupted Dutch translation file: a duplicate "Online now"
  source string broke compilation with msgfmt, and the shipped .mo
  itself wasn't validly sorted - Dutch translations were silently
  failing to load. Rebuilt with msgfmt and added translations for the
  two new strings.