Fix the issue with flags being submitted after the game has stopped

[chloestefantsova]

Here is the stacktrace for the issue.

Internal Server Error: /api/flag/
Traceback (most recent call last):
  File "/srv/parabel.sibctf.ru/bb3-env/local/lib/python2.7/site-packages/django/core/handlers/base.py", line 111, in get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/srv/parabel.sibctf.ru/bb3-env/local/lib/python2.7/site-packages/django/contrib/auth/decorators.py", line 21, in _wrapped_view
    return view_func(request, *args, **kwargs)
  File "/srv/parabel.sibctf.ru/bb3-env/local/lib/python2.7/site-packages/django/views/decorators/csrf.py", line 57, in wrapped_view
    return view_func(*args, **kwargs)
  File "/srv/parabel.sibctf.ru/bb3-env/local/lib/python2.7/site-packages/django/views/generic/base.py", line 69, in view
    return self.dispatch(request, *args, **kwargs)
  File "/srv/parabel.sibctf.ru/bb3-env/local/lib/python2.7/site-packages/rest_framework/views.py", line 452, in dispatch
    response = self.handle_exception(exc)
  File "/srv/parabel.sibctf.ru/bb3-env/local/lib/python2.7/site-packages/rest_framework/views.py", line 449, in dispatch
    response = handler(request, *args, **kwargs)
  File "/srv/parabel.sibctf.ru/blackbox3/api/views.py", line 191, in post
    return Respone({'result': _('Game over!')},
NameError: global name 'Respone' is not defined