Added optional argument to forms.MultipleChoiceField to ignore invalid choices.

[andvodka]

Added optional argument to forms.MultipleChoiceField and forms.ModelMultipleChoiceField to ignore invalid choices.

[felixxm]

@andvodka Thanks for this patch, however, all new features require an accepted ticket.

Please follow the guidelines with regards to requesting features.

[github-actions]

Hello! Thank you for your contribution 💪

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