Enable placeholders in letter contact block

Depends on:
- [x] https://github.com/alphagov/notifications-api/pull/950
- [x] https://github.com/alphagov/notifications-template-preview/pull/18
- [x] https://github.com/alphagov/notifications-utils/pull/161
- [ ] https://github.com/alphagov/notifications-utils/pull/164

app/main/forms.py

@@ -502,7 +502,11 @@ class ServiceSmsSender(Form):
 
 
 class ServiceLetterContactBlock(Form):
-    letter_contact_block = TextAreaField()
+    letter_contact_block = TextAreaField(
+        validators=[
+            NoCommasInPlaceHolders()
+        ]
+    )
 
     def validate_letter_contact_block(form, field):
         line_count = field.data.strip().count('\n')