Make row in URL match displayed row

Spreadsheets start at row 1 (the header row), and the values don’t start
until row 2. The row numbers in our URLs start at 0, which is a concept
that only makes sense to programmers.

It’s more predictable and consistent to make the number in the URL match
the row number displayed on the page when previewing the spreadsheet.

tests/app/main/views/test_send.py

@@ -472,13 +472,13 @@ def test_upload_valid_csv_redirects_to_check_page(
         'Test Service: A, Template <em>content</em> with & entity',
     ),
     (
-        {'row_index': 0},
+        {'row_index': 2},
         None,
         'To: 07700900001',
         'Test Service: A, Template <em>content</em> with & entity',
     ),
     (
-        {'row_index': 2},
+        {'row_index': 4},
         True,
         'To: 07700900003',
         'Test Service: C, Template <em>content</em> with & entity',
@@ -524,7 +524,7 @@ def test_upload_valid_csv_shows_preview_and_table(
 
     if expected_link_in_first_row:
         assert page.select_one('.table-field-index a')['href'] == url_for(
-            'main.check_messages', service_id=SERVICE_ONE_ID, template_type='sms', upload_id=fake_uuid, row_index=0
+            'main.check_messages', service_id=SERVICE_ONE_ID, template_type='sms', upload_id=fake_uuid, row_index=2
         )
     else:
         assert not page.select_one('.table-field-index').select_one('a')
@@ -546,9 +546,12 @@ def test_upload_valid_csv_shows_preview_and_table(
 
 
 @pytest.mark.parametrize('row_index, expected_status', [
-    (0, 200),
+    (0, 404),
+    (1, 404),
     (2, 200),
-    (3, 404),
+    (3, 200),
+    (4, 200),
+    (5, 404),
 ])
 def test_404_for_previewing_a_row_out_of_range(
     client_request,
@@ -1480,11 +1483,11 @@ def test_can_start_letters_job(
         {'postcode': 'abc123', 'addressline1': '123 street'},
     ),
     (
-        {'row_index': 0},
+        {'row_index': 2},
         {'postcode': 'abc123', 'addressline1': '123 street'},
     ),
     (
-        {'row_index': 1},
+        {'row_index': 3},
         {'postcode': 'cba321', 'addressline1': '321 avenue'},
     ),
 ])