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.
You don't have to put optional parameters into the signature like this - the parameter would still be available on the request object as normal. It'd be a little cleaner without but I think this is good for highlighting this feature though so totally fine here.