Add postcode validation check for one-off letters

We’re doing this everywhere else now, so this completes the story.

It uses the same regex as elsewhere and the error messaging is
consistent (but not uniform) with the other places.

app/main/forms.py

@@ -769,6 +769,11 @@ class LetterAddressForm(StripWhitespaceForm):
                 f'Address must be no more than {PostalAddress.MAX_LINES} lines long'
             )
 
+        if not address.postcode:
+            raise ValidationError(
+                f'Last line of the address must be a real UK postcode'
+            )
+
 
 class EmailTemplateForm(BaseTemplateForm):
     subject = TextAreaField(