Let Identity skip confirmation for registration

This is predominantly for Membership, although NGW can opt-in if you like too. Basically, you can append `?skipConfirmation=true` to a link to `https://profile.theguardian.com/register` which, when used in combination with a `returnUrl`, means the user will never be shown this page:

![image](https://cloud.githubusercontent.com/assets/394376/4648045/417493c0-547f-11e4-9c69-a7e770eafed8.png)

Apparently that confirmation page is a requirement for Guardian Jobs and/or Discussion, but for Membership we don't require a confirmed email address (and in any case that button doesn't really do anything except remind people to confirm) and we want to ensure a smooth user journey after registering.

Credit to @wpf500 for help with the Scala here.
I think the argument for Option / Boolean in the `IdRequest` object (of course it's an option when read from the param) is basically "is there a meaningful difference between `None` and `Some(false)`". As I spotted in comment 2, when it's `None` it doesn't appear in the generated URL s which makes sense. Up to you guys.