fix(admin): Removed VMs shows the host a VM left and confirms a delete

A tombstone kept the VM's name and UUID but not its compute, so the page
could not say where the VM had been. It now stores the compute's name (as
text: the compute may be the one deleted) and the list shows it; records
made before this have no host. Deleting a record says so, and the admin
tables use scope="col" instead of the invalid span="col".

Closes #217

admin/views.py

@@ -338,6 +338,7 @@ def tombstone_delete(request, pk):
     if request.method == "POST":
         with audit(request, None, f"Removed VM {tombstone.name} ({tombstone.uuid})", "delete"):
             tombstone.delete()
+        messages.success(request, _("Removed VM %(name)s deleted") % {"name": tombstone.name})
         return redirect("admin:tombstone_list")
 
     return render(