[feature] Allow overriding/customizing widget

[nemesifier]

I have implemented some changes in OpenWISP Controller which flag some m2m relations as required and hence disables the checkbox, so that the m2m relations are forced.

To implement this in the admin widget I had to duplicate a lot of code of sortedm2m, which is not great. This area could be improved with small and backward compatible changes which would allow this kind of change without having to duplicate SortedCheckboxSelectMultiple, 'SortedMultipleChoiceField', SortedManyToManyField.

Is this change something the maintainers are interested in adding? If yes we can send a patch.