Añadir configuración para Django 3.x

The default value of the X_FRAME_OPTIONS setting was changed on Django 3.0 from SAMEORIGIN to DENY.
SummernoteWidget requires 'SAMEORIGIN'.

See

* [Clickjacking Protection](https://docs.djangoproject.com/en/3.0/ref/clickjacking/)
* <https://github.com/summernote/django-summernote/commit/3d44a3ab1758a1a412e9f582bf71be9ec31ee547#diff-04c6e90faac2675aa89e2176d2eec7d8>
parent 489551aa
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