Use Etherpad /health endpoint

Summary:
Etherpad 1.9.0 has introduced a new /health endpoint.

It makes more sense to use this dedicated health check
than /stats coming from a plugin.

The /health response follows the Health Check Response Format
for HTTP APIs specification. Meanwhile, we can use the HTTP 200 code.

Reference: https://www.ietf.org/archive/id/draft-inadarei-api-health-check-06.html

Reviewers: DorianWinty

Reviewed By: DorianWinty

Differential Revision: https://devcentral.nasqueron.org/D3263