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/templates/views/service-settings/set-letter-contact-block.html

@@ -18,7 +18,8 @@
         label='How should users contact your service?<br>This applies to all the letters you send.'|safe,
         hint='10 lines maximum',
         width='1-1',
-        rows=10
+        rows=10,
+        highlight_tags=True
       ) }}
       {{ page_footer(
         'Save',