Admin: add multiselect widget_class for ModelMultipleChoiceField
Admin: add multiselect widget_class for ModelMultipleChoiceField

Add multiselect widget_class for ModelMultipleChoiceFields automatically to
prevent checking field class in templates.

shoopio/shoop,shawnadelic/shuup,shoopio/shoop,akx/shoop,hrayr-artunyan/shuup,shawnadelic/shuup,hrayr-artunyan/shuup,akx/shoop,shoopio/shoop,suutari-ai/shoop,shawnadelic/shuup,jorge-marques/shoop,taedori81/shoop,suutari/shoop,jorge-marques/shoop,akx/shoop,suutari-ai/shoop,suutari/shoop,taedori81/shoop,hrayr-artunyan/shuup,suutari-ai/shoop,suutari/shoop,taedori81/shoop,jorge-marques/shoop