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_views.py

@@ -126,6 +126,10 @@ class ConsoleViewsTestCase(TestCase):
 
         self.assertEqual(response.status_code, 200)
         self.assertTemplateUsed(response, "console-xterm.html")
+        # the page's credential for socketiod, as data
+        signed = json.loads(html.fromstring(response.content).get_element_by_id("serial-token").text)
+        self.assertEqual(TimestampSigner(salt="console.novnc").unsign(signed),
+                         f"{self.compute.id}:{self.instance.uuid}:{self.admin_user.id}")
 
     @override_settings(SERIAL_CONSOLE_ENABLED=False)
     @patch("console.views.wvmInstance")