Add a page for each report

It’s useful to get some kind of preview of the report before you
download it.

And if there’s only a few letters in there then you might not even need
to download it at all.

For teams with lots of letters we don’t want the page to load too slowly
so let’s cap the number of displayed items to 50, same as previewing
a spreadsheet.

app/navigation.py

@@ -243,6 +243,7 @@ class HeaderNavigation(Navigation):
         'resend_email_verification',
         'resume_service',
         'returned_letter_summary',
+        'returned_letters',
         'returned_letters_report',
         'revoke_api_key',
         'robots',
@@ -567,6 +568,7 @@ class MainNavigation(Navigation):
         'resend_email_verification',
         'resume_service',
         'returned_letter_summary',
+        'returned_letters',
         'returned_letters_report',
         'roadmap',
         'robots',
@@ -806,6 +808,7 @@ class CaseworkNavigation(Navigation):
         'resend_email_verification',
         'resume_service',
         'returned_letter_summary',
+        'returned_letters',
         'returned_letters_report',
         'revoke_api_key',
         'roadmap',
@@ -1084,6 +1087,7 @@ class OrgNavigation(Navigation):
         'resend_email_verification',
         'resume_service',
         'returned_letter_summary',
+        'returned_letters',
         'returned_letters_report',
         'revoke_api_key',
         'roadmap',