Use nginx instead of webrick when hosting on Heroku (#156)

This uses the heroku NGINX buildpack to serve up the static files instead of a ruby process.

This is better in prod than webrick, as it is a proper webserver that can handle more than one request at a time. This will do while we sort out Cloudflare buckets etc.

Procfile

@@ -1 +1 @@
-web: ruby -run -e httpd -- -p $PORT build/
+web: bin/start-nginx-solo

app.json

@@ -8,6 +8,9 @@
   "buildpacks": [
     {
       "url": "heroku/nodejs"
+    },
+    {
+      "url": "https://github.com/heroku/heroku-buildpack-nginx"
     }
   ]
 }

config/nginx.conf.erb

@@ -0,0 +1,39 @@
+daemon off;
+# Heroku dynos have at least 4 cores.
+worker_processes <%= ENV['NGINX_WORKERS'] || 4 %>;
+
+events {
+	use epoll;
+	accept_mutex on;
+	worker_connections <%= ENV['NGINX_WORKER_CONNECTIONS'] || 1024 %>;
+}
+
+http {
+	gzip on;
+	gzip_comp_level 2;
+	gzip_min_length 512;
+	gzip_proxied any; # Heroku router sends Via header
+
+	server_tokens off;
+
+	log_format l2met 'measure#nginx.service=$request_time request_id=$http_x_request_id';
+	access_log <%= ENV['NGINX_ACCESS_LOG_PATH'] || 'logs/nginx/access.log' %> l2met;
+	error_log <%= ENV['NGINX_ERROR_LOG_PATH'] || 'logs/nginx/error.log' %>;
+
+
+	include mime.types;
+	default_type application/octet-stream;
+	sendfile on;
+
+	# Must read the body in 5 seconds.
+	client_body_timeout <%= ENV['NGINX_CLIENT_BODY_TIMEOUT'] || 5 %>;
+
+	server {
+		listen <%= ENV["PORT"] %>;
+		server_name _;
+		keepalive_timeout 5;
+		client_max_body_size <%= ENV['NGINX_CLIENT_MAX_BODY_SIZE'] || 1 %>M;
+
+		root /app/build; # path to your app
+	}
+}