Add support for proxying /_synapse/admin APIs

Fixes #191 (Github Issue).

roles/matrix-nginx-proxy/templates/nginx/conf.d/matrix-synapse.conf.j2

@@ -169,6 +169,25 @@ server {
 	}
 	{% endif %}
 
+	location /_synapse/admin {
+		{% if matrix_nginx_proxy_enabled %}
+			{# Use the embedded DNS resolver in Docker containers to discover the service #}
+			resolver 127.0.0.11 valid=5s;
+			set $backend "{{ matrix_nginx_proxy_proxy_matrix_client_api_addr_with_container }}";
+			proxy_pass http://$backend;
+		{% else %}
+			{# Generic configuration for use outside of our container setup #}
+			proxy_pass http://{{ matrix_nginx_proxy_proxy_matrix_client_api_addr_sans_container }};
+		{% endif %}
+
+		proxy_set_header Host $host;
+		proxy_set_header X-Forwarded-For $remote_addr;
+
+		client_body_buffer_size 25M;
+		client_max_body_size {{ matrix_nginx_proxy_proxy_matrix_client_api_client_max_body_size_mb }}M;
+		proxy_max_temp_file_size 0;
+	}
+
 	location / {
 		rewrite ^/$ /_matrix/static/ last;
 	}