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/socketiod

@@ -75,6 +75,9 @@ def connect(sid, environ, auth):
             raise refused("Too many consoles are open on this server: try again later")
         if open_sid is not None:  # a superuser takes over a console left open
             taken = open_sid, sessions.pop(open_sid)
+            # the page tells its user who took the console; first, as its
+            # forward ends the connection as soon as the worker is gone
+            sio.emit("taken_over", {"by": user.username}, to=open_sid)
             # gone before the new worker starts: libvirt gives a console to one at a time
             end(taken[1], wait=True)
         pid, fd = pty.fork()
@@ -96,9 +99,7 @@ def connect(sid, environ, auth):
     logger.info("serial console of VM %s opened by %s%s", instance.name, user.username,
                 " (taken over)" if taken else "")
     if taken:
-        old_sid, _ = taken
-        sio.emit("pty_output", {"output": "\r\n[An administrator opened this console]\r\n"}, to=old_sid)
-        sio.disconnect(old_sid)
+        sio.disconnect(taken[0])
     sio.start_background_task(forward, sid, session)