Web API: add a /ping endpoint

It would be nice to have a /ping API endpoint, as a mnemonic one to test if the API is working, the current rate limit, etc. The endpoint would do nothing except return "pong" (what else, right? :-))

Our documentation currently uses /stat/counters as an example in similar cases. /ping sounds nicer and could be used instead of that.

---

Migrated from T2294 (view on Phabricator)