rate-limit requests for rss feed

someone has made a conky script that hits the rss feed every second, so nginx
now limits that to once per minute, returning 503 instead at other times. a
burst is also in place to prevent genuine users being blocked.

roles/fluxbb/templates/nginx.conf.j2

@@ -13,6 +13,11 @@ server {
     }
 }
 
+# a limiter to stop abuse of the rss feed.
+# limit to 1 requests per minute, with a burst defined when we use this
+# limiter in the location directive below
+limit_req_zone $binary_remote_addr zone=rsslimit:8m rate=1r/m;
+
 server {
     listen 443 ssl http2;
     listen [::]:443 ssl http2;
@@ -27,6 +32,10 @@ server {
     ssl_certificate_key  /etc/letsencrypt/live/{{ fluxbb_domain }}/privkey.pem;
     ssl_trusted_certificate /etc/letsencrypt/live/{{ fluxbb_domain }}/chain.pem;
 
+    location ~ /extern\.php {
+        limit_req zone=rsslimit burst=10 nodelay;
+    }
+
     location ~ ^/(?:config|header|footer)\.php {
         log_not_found off;
         deny all;