Make the letter-validation-preview url public.

In the short term I have created a duplicate version of the letter-validation-preview so that people from a service can upload a pdf and see why the letter isnot validating.

It's hard to get a precompiled letter to validate when starting to integrate with Notify. This will return the overlay of the letter validation and is now available to the services.
At the moment they send us a PDF to upload.
This is temporary because there is a story to create a one-off flow to get this overlay, that will replace this page.

There is no navigation to this on purpose.

app/navigation.py

@@ -233,6 +233,7 @@ class HeaderNavigation(Navigation):
         'service_edit_sms_sender',
         'service_email_reply_to',
         'service_letter_contact_details',
+        'service_letter_validation_preview',
         'service_name_change',
         'service_name_change_confirm',
         'service_preview_email_branding',
@@ -487,6 +488,7 @@ class MainNavigation(Navigation):
         'service_dashboard_updates',
         'service_delete_email_reply_to',
         'service_delete_sms_sender',
+        'service_letter_validation_preview',
         'service_switch_can_upload_document',
         'service_switch_live',
         'service_set_permission',
@@ -701,6 +703,7 @@ class CaseworkNavigation(Navigation):
         'service_edit_sms_sender',
         'service_email_reply_to',
         'service_letter_contact_details',
+        'service_letter_validation_preview',
         'service_name_change',
         'service_name_change_confirm',
         'service_preview_email_branding',
@@ -940,6 +943,7 @@ class OrgNavigation(Navigation):
         'service_edit_sms_sender',
         'service_email_reply_to',
         'service_letter_contact_details',
+        'service_letter_validation_preview',
         'service_name_change',
         'service_name_change_confirm',
         'service_preview_email_branding',