Fix exception related to page data on server start

What does this MR do and why?

Add background key to page data to avoid stack in logs on server start.

Screenshots or screen recordings

2023-01-31 13:10:09,990 - [Crafty] - ERROR    - tornado.application - Uncaught exception POST /server/command?command=start_server&id=1 (127.0.0.1)
HTTPServerRequest(protocol='https', host='127.0.0.1:8443', method='POST', uri='/server/command?command=start_server&id=1', version='HTTP/1.1', remote_ip='127.0.0.1')
Traceback (most recent call last):
  File "C:\Users\Documents\crafty-4\venv\lib\site-packages\tornado\web.py", line 1697, in _execute
    result = method(*self.path_args, **self.path_kwargs)
  File "C:\Users\Documents\crafty-4\venv\lib\site-packages\tornado\web.py", line 3174, in wrapper
    return method(self, *args, **kwargs)
  File "C:\Users\Documents\crafty-4\app\classes\web\server_handler.py", line 597, in post
    self.render(
  File "C:\Users\Documents\crafty-4\venv\lib\site-packages\tornado\web.py", line 856, in render
    html = self.render_string(template_name, **kwargs)
  File "C:\Users\Documents\crafty-4\venv\lib\site-packages\tornado\web.py", line 1005, in render_string
    return t.generate(**namespace)
  File "C:\Users\Documents\crafty-4\venv\lib\site-packages\tornado\template.py", line 361, in generate
    return execute()
  File "public/404_html.generated.py", line 5, in _tt_execute
    _tt_tmp = data['background']  # public/404.html:26
KeyError: 'background'

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