feat(console): the serial console page on the panel's bar; a taken-over user learns by whom

The serial page had a white strip of inline styles with an untranslated
status and a browser-default button above the terminal, sent input while
disconnected, sized the terminal only on window load and resize, and left
the keyboard elsewhere after the key menu. When a superuser opened a
console open in another window, its user saw "[An administrator opened
this console]" and a plain "disconnected".

- console-xterm.html shares the VNC page's bar: status badge (Connecting,
  Connected, Disconnected, the server's refusal), Disconnect, then
  Reconnect (a new page for a new token). The script is
  static/js/console_serial.js; the token reaches it as json_script data.
- The terminal takes the keyboard on connect and after the key menu; input
  is sent only while connected; it fits its box on connect and whenever
  the box changes size (ResizeObserver).
- A console that writes nothing yet gets a dim local hint: press Enter.
- socketiod sends the displaced client "taken_over" with the superuser's
  username; the page shows "Taken over by <name>" in the bar and the
  terminal. The superuser is not asked first (owner's decision).
- Fullscreen uses the shared console_keys.js handler (keyboard lock).

console/tests/test_socketiod_child.py

@@ -87,8 +87,9 @@ def start_socketiod(test):
 def console(port, token):
     """A connected client and what it receives until the server disconnects it."""
     client = socketio.Client(reconnection=False)
-    received = {"output": [], "gone": threading.Event()}
+    received = {"output": [], "taken_over": [], "gone": threading.Event()}
     client.on("pty_output", lambda data: received["output"].append(data["output"]))
+    client.on("taken_over", lambda data: received["taken_over"].append(data["by"]))
     client.on("disconnect", lambda *args: received["gone"].set())
     # the same origin as the server, as the page through nginx
     url = "http://127.0.0.1:%d" % port