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.

app/templates/views/check/ok.html

@@ -60,10 +60,10 @@
       ) %}
         {% call index_field() %}
           <span>
-            {% if item.index == preview_row %}
+            {% if (item.index + 2) == preview_row %}
               {{ item.index + 2 }}
             {% else %}
-              <a href="{{ url_for('.check_messages', service_id=current_service.id, template_type=template.template_type, upload_id=upload_id, row_index=(item.index)) }}">{{ item.index + 2 }}</a>
+              <a href="{{ url_for('.check_messages', service_id=current_service.id, template_type=template.template_type, upload_id=upload_id, row_index=(item.index + 2)) }}">{{ item.index + 2 }}</a>
             {% endif %}
           </span>
         {% endcall %}