Move inactive / legacy members of IO to other Credits page

[duckth]

The credits page currently includes old members of IO that are no longer part of the development team.
I think we should keep the Credits page as only active members as it gives a clearer indication of who is currently working on the project, and who you can contact from the team.
However, legacy members of course deserve credit too, so I suggest we add some kind of "hall of fame" or "contributors" page separate to the main Credits page, that lists all of our lovely contributors, active or not.