Fix issues in friendly-timestamp-string, add statusz

* Added statusz route
* `friendly-timestamp-string` was intermittently breaking on nil values (I couldn't determine why it wasn't happening consistently, it was very infrequent). I added a guard against nil values to prevent these.
* `friendly-timestamp-string` would display "0 days ago" when time ago was between 12 and 24 hours ago. I changed it to display "hours ago" until 24 hours.
We could also just serve this file directly from nginx and keep it out of the watchman repo entirely.