Add a platform admin page to submit returned letter references

A platform admin form accepts a list of references (one per line)
received from DVLA and sends them to the API to update notification
statuses.

References we get from DVLA start with `NOTIFY00\d`, which isn't
part of the reference we store in the database, so we remove them
before sending the data to the API.

The new `returned-letter` status should be treated as `delivered`
for now until we decide a way to display returned letters to users.

app/navigation.py

@@ -81,6 +81,7 @@ class HeaderNavigation(Navigation):
             'organisations',
             'platform_admin',
             'platform_admin_list_complaints',
+            'platform_admin_returned_letters',
             'suspend_service',
             'trial_services',
             'update_email_branding',
@@ -440,6 +441,7 @@ class MainNavigation(Navigation):
         'organisations',
         'platform_admin',
         'platform_admin_list_complaints',
+        'platform_admin_returned_letters',
         'pricing',
         'privacy',
         'public_agreement',
@@ -624,6 +626,7 @@ class CaseworkNavigation(Navigation):
         'organisations',
         'platform_admin',
         'platform_admin_list_complaints',
+        'platform_admin_returned_letters',
         'pricing',
         'privacy',
         'public_agreement',
@@ -848,6 +851,7 @@ class OrgNavigation(Navigation):
         'organisations',
         'platform_admin',
         'platform_admin_list_complaints',
+        'platform_admin_returned_letters',
         'pricing',
         'privacy',
         'public_agreement',