Add Nginx header to not buffer SSE connection

This should allow the Activity Panel, that uses a Server-Side Events/ Event Source connection, to work with default Nginx reverse proxy configuration.

server/events/sse.go

@@ -120,6 +120,8 @@ func (b *broker) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	w.Header().Set("Cache-Control", "no-cache, no-transform")
 	w.Header().Set("Connection", "keep-alive")
 	w.Header().Set("Access-Control-Allow-Origin", "*")
+	// Tells Nginx to not buffer this response. See https://stackoverflow.com/a/33414096
+	w.Header().Set("X-Accel-Buffering", "no")
 
 	// Each connection registers its own message channel with the Broker's connections registry
 	c := b.subscribe(r)