Show correct period start and period end in invoice list

Previously, we used to take the first element in the bill line
item, for invoice start and end date. However, it can be faulty in
cases where the first item does not represent a VM itself (which
we are sure has a monthly subscription and a correct start and end
dates)

hosting/models.py

@@ -521,6 +521,7 @@ class HostingBillLineItem(AssignPermissionsMixin, models.Model):
                 )
         return item_detail
 
+
 class VMDetail(models.Model):
     user = models.ForeignKey(CustomUser)
     vm_id = models.IntegerField(default=0)