Show checkboxes on email signup

![screenshot](https://www.dropbox.com/s/ojtf6pvuonhk3li/Screenshot%202014-11-10%2011.13.47.png?dl=1)

* Redisplay the email signup input box.
* For each choice in email signup choices, render a checkbox with the appropriate choice value.
* Style the checkboxes the same as the finder checkboxes for now -- @alicebartlett This is probably worth a frontend review :)

This depends on https://github.com/alphagov/finder-api/pull/83
Further to @bradleywright's comment, have a look at [GOV.UK Elements](http://govuk-elements.herokuapp.com/#guide-forms) for more info on how to make forms that are consistent with the rest of GOV.UK.