fix: Ensure that Box content is rendered after update

[ewdurbin]

This resolves the underlying problem from #2631.

When updated via Django's update_or_create, the new content for a Box is not rendered by django-markupfield's pre_save method unless save() is called.

This is due to the fact that update bypasses calls to save, pre_save, and post_save, from the docs:

Finally, realize that update() does an update at the SQL level and, thus, does not call any save() methods on your models, nor does it emit the pre_save or post_save signals (which are a consequence of calling Model.save())

[ewdurbin]

Summary here is that since the /boxes view and templatetag sensibly use Box.content.rendered and not Box.content, the pre_save method is critical to ensuring that the value passed to content is actually... well... rendered.