fix(console): a VNC token works only in its user's session; refusals reach the Logs page

The VNC page's token travels in the websocket URL, so nginx's access log
and anyone who saw the URL had it for 120 s, and it opened the console as
a bearer token. novncd now also wants the session cookie of the token's
user. Its refusals were a warning without a user or an address: they now
name the client address (nginx sends X-Real-IP to novncd too) and leave
a Logs row "VNC console refused: reason".

console/tokens.py

@@ -1,11 +1,18 @@
 """The console token: the console view signs "host:uuid:user_id:console type"
 for the VM whose console it shows; the console daemons (novncd for vnc,
 socketiod for pty) accept it for 120 s if that user may still open that VM's
-console. A bearer token: whoever holds it within that time gets the console."""
+console. novncd, whose token travels in the websocket URL (and so in logs),
+also wants the session cookie of the token's user (session_user_id).
+socketiod's token travels in the Socket.IO handshake, not the URL."""
+
+from importlib import import_module
 
 from django import db
+from django.conf import settings
+from django.contrib.auth import SESSION_KEY
 from django.contrib.auth import get_user_model
 from django.core.signing import BadSignature, SignatureExpired, TimestampSigner
+from django.http import parse_cookie
 
 from instances.models import Instance
 from instances.utils import can_open_console
@@ -44,3 +51,12 @@ def may_still_open(user_id, instance):
         return user is not None and can_open_console(user, instance)
     finally:
         db.connection.close()  # a session may last for days: no idle connection meanwhile
+
+
+def session_user_id(cookie_header):
+    """The id (a string) of the user signed in with the session cookie of a
+    request's Cookie header, or None."""
+    session_key = parse_cookie(cookie_header or "").get(settings.SESSION_COOKIE_NAME)
+    if not session_key:
+        return None
+    return import_module(settings.SESSION_ENGINE).SessionStore(session_key).get(SESSION_KEY)