Handle client list size changing on websocket send

What does this MR do and why?

Fix Crafty crash for client list changing while sending websockets

Screenshots or screen recordings

Exception in thread command_watcher:
Traceback (most recent call last):
  File "/usr/lib/python3.9/threading.py", line 954, in _bootstrap_inner
    self.run()
  File "/usr/lib/python3.9/threading.py", line 892, in run
    self._target(*self._args, **self._kwargs)
  File "/var/opt/minecraft/crafty/crafty-4/app/classes/shared/tasks.py", line 118, in command_watcher
    svr.restart_threaded_server(user_id)
  File "/var/opt/minecraft/crafty/crafty-4/app/classes/shared/server.py", line 674, in restart_threaded_server
    self.stop_threaded_server()
  File "/var/opt/minecraft/crafty/crafty-4/app/classes/shared/server.py", line 584, in stop_threaded_server
    self.stop_server()
  File "/var/opt/minecraft/crafty/crafty-4/app/classes/shared/server.py", line 660, in stop_server
    self.helper.websocket_helper.broadcast_user(user, "send_start_reload", {})
  File "/var/opt/minecraft/crafty/crafty-4/app/classes/web/websocket_helper.py", line 49, in broadcast_user
    self.broadcast_with_fn(filter_fn, event_type, data)
  File "/var/opt/minecraft/crafty/crafty-4/app/classes/web/websocket_helper.py", line 88, in broadcast_with_fn
    clients = list(filter(filter_fn, self.clients))
RuntimeError: Set changed size during iteration

MR acceptance checklist

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• Have you resolved any lint issues?
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