Allow including custom nginx conf files (#178)

* Allow including custom nginx conf files

Give advanced users more flexibility by allowing them to include custom config files at differents locations in the nginx configuration.

`/data/nginx/custom/root.conf`: Included at the very end of nginx.conf
`/data/nginx/custom/http.conf`: Included at the end of the main `http` block
`/data/nginx/custom/server_proxy.conf`: Included at the end of every proxy `server` block
`/data/nginx/custom/server_redirect.conf`: Included at the end of every redirection `server` block
`/data/nginx/custom/server_stream.conf`: Included at the end of every stream `server` block
`/data/nginx/custom/server_stream_tcp.conf`: Included at the end of every TCP stream `server` block
`/data/nginx/custom/server_stream_udp.conf`: Included at the end of every UDP stream `server` block

* Don't fail if file doesn't exist

* Advanced Nginx settings doc

src/backend/templates/proxy_host.conf

@@ -41,5 +41,7 @@ server {
   }
 {% endif %}
 
+  # Custom
+  include /data/nginx/custom/server_proxy[.]conf;
 }
 {% endif %}

src/backend/templates/redirection_host.conf

@@ -25,5 +25,7 @@ server {
   }
 {% endif %}
 
+  # Custom
+  include /data/nginx/custom/server_redirect[.]conf;
 }
 {% endif %}

src/backend/templates/stream.conf

@@ -7,12 +7,20 @@
 server {
   listen {{ incoming_port }};
   proxy_pass {{ forward_ip }}:{{ forwarding_port }};
+
+  # Custom
+  include /data/nginx/custom/server_stream[.]conf;
+  include /data/nginx/custom/server_stream_tcp[.]conf;
 }
 {% endif %}
 {% if udp_forwarding == 1 or udp_forwarding == true %}
 server {
   listen {{ incoming_port }} udp;
   proxy_pass {{ forward_ip }}:{{ forwarding_port }};
+
+  # Custom
+  include /data/nginx/custom/server_stream[.]conf;
+  include /data/nginx/custom/server_stream_udp[.]conf;
 }
 {% endif %}
 {% endif %}