Add extra text to label rows for screenreaders

This is mainly for the ‘check’ page where we show users the contents of
their spreadsheet.

> The structure of the table means that the first cell is treated as a
> column header, so moving through columns in row 2 for example
> associates the data 2 with 1 (column header) - this has no logical
> meaning

app/templates/components/table.html

@@ -73,7 +73,7 @@
 
 {% macro index_field(text) -%}
   <td class="table-field-index">
-    <span>{{ text }}</span>
+    <span><span class="visually-hidden">Row </span>{{ text }}</span>
   </td>
 {%- endmacro %}