Add template name field to invoice group

aleksis/apps/tezor/models/invoice.py

@@ -18,6 +18,8 @@ class InvoiceGroup(ExtensibleModel):
         Client, verbose_name=_("Linked client"), related_name="invoice_groups"
     )
 
+    template_name = models.CharField(verbose_name=_("Template to render invoices with as PDF"), blank=True)
+
     class Meta:
         constraints = [
             models.UniqueConstraint(fields=["client", "name"], name="group_uniq_per_client")