Use better column heading for screenreaders
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.

alphagov/notifications-admin,alphagov/notifications-admin,alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin,gov-cjwaszczuk/notifications-admin,alphagov/notifications-admin,gov-cjwaszczuk/notifications-admin,gov-cjwaszczuk/notifications-admin