Add clean_value in FlagStateForm

[Weltraum]

An sad mistake in the name broke the application, I wrote Flase instead of False:

  File "/Users/vlad/.pyenv/versions/worldskils50+/lib/python3.6/site-packages/flags/state.py", line 46, in flag_enabled
    return flag_state(flag_name, **kwargs)
  File "/Users/vlad/.pyenv/versions/worldskils50+/lib/python3.6/site-packages/flags/state.py", line 41, in flag_state
    return _flag_state(flag_name, **kwargs)
  File "/Users/vlad/.pyenv/versions/worldskils50+/lib/python3.6/site-packages/flags/state.py", line 28, in _flag_state
    return flag.check_state(**kwargs)
  File "/Users/vlad/.pyenv/versions/worldskils50+/lib/python3.6/site-packages/flags/sources.py", line 56, in check_state
    (c, c.check(**kwargs)) for c in self.conditions
  File "/Users/vlad/.pyenv/versions/worldskils50+/lib/python3.6/site-packages/flags/sources.py", line 56, in <listcomp>
    (c, c.check(**kwargs)) for c in self.conditions
  File "/Users/vlad/.pyenv/versions/worldskils50+/lib/python3.6/site-packages/flags/sources.py", line 28, in check
    return self.fn(self.value, **kwargs)
  File "/Users/vlad/.pyenv/versions/worldskils50+/lib/python3.6/site-packages/flags/conditions.py", line 62, in boolean_condition
    return strtobool(condition.strip().lower())
  File "/Users/vlad/.pyenv/versions/3.6.10/lib/python3.6/distutils/util.py", line 317, in strtobool
    raise ValueError("invalid truth value %r" % (val,))
ValueError: invalid truth value 'flase'

This happened because I placed the check flag_enabled('MY_FLAG') in middleware.

I have now redefined the class FlagStateForm:

class CleanFlagStateForm(FlagStateForm):

    def clean_value(self):
        condition = self.cleaned_data.get('condition')
        value = self.cleaned_data.get('value')
        try:
            CONDITIONS[condition](value)
        except Exception as e:
            raise forms.ValidationError(e)
        return value

But it seems to me that this check should be in the library itself