Historical roles link to history section

Tracker: https://www.pivotaltracker.com/story/show/48691569

On the role page for Prime Minister, instead of listing the 70-odd previous appointments that now exist, we link directly to the history section for that role.

Note that this work assumes that the existing PM role is renamed from "Prime Minister, First Lord of the Treasury and Minister for the Civil Service" to simply "Prime Minister", and that separate roles are created for "First Lord of the Treasury" and "Minister for the Civil Service". Waiting on confirmation from content on that being done.