Use better column heading for screenreaders

When a screenreader user navigates a table, they use the columns
headings to orientate themselves. A column heading of ‘1’ is not
helpful.

So this commit adds some hidden text for screenreader users, which tells
them exactly what the column contains: the number of the row in the
original file.

app/templates/views/check.html

@@ -170,7 +170,9 @@
     recipients.initial_annotated_rows_with_errors if row_errors and not recipients.missing_column_headers else recipients.initial_annotated_rows,
     caption=original_file_name,
     caption_visible=False,
-    field_headings=['1'] + recipients.column_headers
+    field_headings=[
+      '<span class="visually-hidden">Row in file</span><span aria-hidden="true">1</span>'|safe
+    ] + recipients.column_headers
   ) %}
     {{ index_field(item.index + 2) }}
     {% for column in recipients.column_headers %}