feat(logs): each log line names its request, user and client address

A request gets nginx's $request_id (or a new one when it comes without a
valid one) and sends it back in X-Request-ID. The app log's lines carry
the logger, process and thread, the request ID, the signed-in user and the
client address; gunicorn's access lines carry the client address from
X-Real-IP and the same request ID, so the two logs can be joined.

conf/nginx/webvirtcloud.conf

@@ -25,6 +25,7 @@ server {
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
         proxy_set_header Host $http_host;
         proxy_set_header X-Forwarded-Proto $wvc_forwarded_proto;
+        proxy_set_header X-Request-ID $request_id;
         proxy_connect_timeout 1800;
         proxy_read_timeout 1800;
         proxy_send_timeout 1800;