Fixes #3898: Call str of cable on delete to save PK in id_string

[hSaria]

Fixes: #3898

Called the __str__ method of the cable to create the self.id_string attribute if not there. An alternative solution would've been to move the msg line before delete() is called, but I checked all models' __str__ method and it's only Cable that uses the PK as part of the string.

netbox/netbox/utilities/views.py

Lines 289 to 295 in b7e7802

	try:
	obj.delete()
	except ProtectedError as e:
	handle_protectederror(obj, request, e)
	return redirect(obj.get_absolute_url())
	msg = 'Deleted {} {}'.format(self.model._meta.verbose_name, obj)

[jeremystretch]

My fix for this under #2984 was admittedly naive, since it requires calling str() on the cable instance first. We can do this more efficiently by setting self._pk = self.pk on init, and having __str__ return self.label or self._pk.

[hSaria]

Yep, that'd be cleaner. Let me adjust the PR.