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 agree it would make more sense as a `Boolean`, but we made it an `Option[Boolean]` just to stay consistent with the rest of the code. In `IdentityUrlBuilder.queryParams` it maps over the `Option[_]`s in `params` and I thought it was better to be consistent and stick to that pattern. If I was being facetious I might argue that stripping out `skipConfirmation=false` from the URL is a bit strange but I don't really mind either way