security: Support proxy with rate limiting and include CI test coverage for nginx rev proxy (#4373)

Previously Etherpad would not pass the correct client IP address through and this caused the rate limiter to limit users behind reverse proxies.  This change allows Etherpad to use a client IP passed from a reverse proxy.

Note to devs: This header can be spoofed and spoofing the header could be used in an attack.  To mitigate additional *steps should be taken by Etherpad site admins IE doing rate limiting at proxy.*  This only really applies to large scale deployments but it's worth noting.

tests/ratelimit/Dockerfile.nginx

@@ -0,0 +1,2 @@
+FROM nginx
+COPY ./tests/ratelimit/nginx.conf /etc/nginx/nginx.conf