Allow use of can_delete flag directly in inline definition

[jgregory-valence]

The previous README had instructions about preventing the deletion of inlined models by overriding the has_delete_permission method on a custom subclass of the appropriate StackedInline or TabularInline class, and then passing that subclass as the admin_class option of inline_reverse. However, that method no longer works with newer versions of Django (at least with Django 4.2 and later).

This commit updates the ReverseInlineModelAdmin class to determine can_delete in the same way that Django's InlineModelAdmin currently does, and then passes that into the custom formset factory. In that way, consumers of this library can simply set the can_delete option directly on their inline_reverse specification without subclassing one of the built-in InlineModelAdmin implementations.

Note that this approach was necessary because the custom formset factory in django_reverse_admin lives outside of the ReverseInlineModelAdmin class, and doesn't have access to the parent class' can_delete property and has_delete_permission method.