replace Person, Position and Organisation models with the components client

[mhl]

We should now be able to use the components application (on the popit branch) to replace handling of Person, Position and Organisation objects with their PopIt-backed equivalents, and remove the options to edit these in the admin interface.

[duncanparkes]

I think this ticket has been overtaken by events, and that #1594, is now as close as we're going to get to this.