Revert "Use 5 calendar days for invoice due date"

This reverts commit 9d5c877f54.

invoicer.py

@@ -77,7 +77,13 @@ class InvoiceInputs:
 
     @property
     def due_date(self) -> date:
-        return self.issue_date + timedelta(days=5)
+        d = self.issue_date
+        added = 0
+        while added < 5:
+            d += timedelta(days=1)
+            if d.weekday() < 5:
+                added += 1
+        return d
 
 
 def prompt_str(label: str, validator=None) -> str: