[cleanup] Avoid sortedm2m code duplication for required templates

[nemesifier]

The required templates feature depends on some modification of django-sortedm2m.

This duplication can be avoided by sending a small patch to django-sortedm2m which includes a test and a brief explanation in the README of how to customize the checbkox widget.

[nemesifier]

The solution I think is to manage required templates only with JS and get rid of this sortedm2m duplication.