Improve presentation of inactive roles

This improves the modelling around roles that are no longer active. The `Role` model now has a status, which can be either: "active"; "no_longer_exists"; "replaced"; "split"; or "merged". Roles that have been marked as replaced, split or merged can have one/many superseded organisation(s). The public-facing role pages for ministerial roles show a status message based on this status.

https://www.pivotaltracker.com/story/show/70175686