Increase max body size for the nginx proxy

Otherwise, we can't support large media file uploads.

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

@@ -33,5 +33,11 @@ server {
 	location /_matrix {
 		proxy_pass http://synapse:8008;
 		proxy_set_header X-Forwarded-For $remote_addr;
+
+		# These are intentionally large.
+		# Note that Synapse enforces its own limits, which are smaller.
+		client_body_buffer_size 25M;
+		client_max_body_size 100M;
+		proxy_max_temp_file_size 0;
 	}
 }