Allow deleting default letter contact blocks

It’s possible to delete default letter contact blocks because there is a
fallback – having a blank letter contact block. This is different to SMS
senders and reply to addresses.

For this to make sense it also means:
- adding the ‘blank’ letter contact block to the list of letter contact
  blocks
- having a way of setting the default back to being blank

app/navigation.py

@@ -260,6 +260,7 @@ class HeaderNavigation(Navigation):
         'service_email_reply_to',
         'service_letter_contact_details',
         'service_letter_validation_preview',
+        'service_make_blank_default_letter_contact',
         'service_name_change',
         'service_name_change_confirm',
         'service_preview_email_branding',
@@ -390,6 +391,7 @@ class MainNavigation(Navigation):
             'service_edit_sms_sender',
             'service_email_reply_to',
             'service_letter_contact_details',
+            'service_make_blank_default_letter_contact',
             'service_name_change',
             'service_name_change_confirm',
             'service_preview_email_branding',
@@ -787,6 +789,7 @@ class CaseworkNavigation(Navigation):
         'service_email_reply_to',
         'service_letter_contact_details',
         'service_letter_validation_preview',
+        'service_make_blank_default_letter_contact',
         'service_name_change',
         'service_name_change_confirm',
         'service_preview_email_branding',
@@ -1059,6 +1062,7 @@ class OrgNavigation(Navigation):
         'service_email_reply_to',
         'service_letter_contact_details',
         'service_letter_validation_preview',
+        'service_make_blank_default_letter_contact',
         'service_name_change',
         'service_name_change_confirm',
         'service_preview_email_branding',