Add rel="alternate" API links from organisations

(Requested by @jystewart)

I don't entirely understand why these links appear in the `<head>` when the frontend base layout only yields in the `<body>` - I think it's Slimmer doing magic with [TagMover](https://github.com/alphagov/slimmer/blob/master/lib/slimmer/processors/tag_mover.rb), but if anyone can explain a bit more, I'd be grateful.