Add inflections to pluralize historic roles
Make the sub-heading consistent with index page

It makes little sense to repeat the role here anyway.
If the role is historic, link to history section

Instead of listing a long list of previous appointments for historic
roles (i.e. the PM role), we link directly to the history page for
that role.

This also reverses the order in which previous appointments are
listed so they are listed in descending order.