Link back to uploaded letters page

If you’ve come to look at a notification via the uploaded letters page
then the ‘< back’ link should take you back there, not to the usual
activity page.

tests/app/main/views/test_uploads.py

@@ -160,6 +160,10 @@ def test_get_uploaded_letters(
         service_id=SERVICE_ONE_ID,
         letter_print_day='2020-02-02'
     )
+    assert page.select_one('.govuk-back-link')['href'] == url_for(
+        'main.uploads',
+        service_id=SERVICE_ONE_ID,
+    )
     assert normalize_spaces(
         page.select_one('h1').text
     ) == (
@@ -192,6 +196,23 @@ def test_get_uploaded_letters(
         ),
     ]
 
+    assert [
+        link['href'] for link in page.select('tbody tr a')
+    ] == [
+        url_for(
+            'main.view_notification',
+            service_id=SERVICE_ONE_ID,
+            notification_id='03e34025-be54-4d43-8e6a-fb1ea0fd1f29',
+            from_uploaded_letters='2020-02-02',
+        ),
+        url_for(
+            'main.view_notification',
+            service_id=SERVICE_ONE_ID,
+            notification_id='fc090d91-e761-4464-9041-9c4594c96a35',
+            from_uploaded_letters='2020-02-02',
+        ),
+    ]
+
     next_page_link = page.select_one('a[rel=next]')
     prev_page_link = page.select_one('a[rel=previous]')
     assert next_page_link['href'] == url_for(