Add /_stats endpoint

Also, remove ingest_stats worker and use the cache instead.

client/utils/formatNumber.js

@@ -0,0 +1,11 @@
+const formatNumber = (n) => {
+  const digits = String(n).split('')
+  const groups = []
+
+  while (digits.length)
+    groups.unshift(digits.splice(-3).join(''))
+
+  return groups.join(',')
+}
+
+export default formatNumber