Show checkboxes on email signup

![screenshot](https://www.dropbox.com/s/9fx6qlgyvo26z5p/Screenshot%202014-11-10%2015.06.29.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

* [x] Merge finder-api/pull/83
* [x] Update JS to add selected/focus states
* [ ] get content review of the page again for the fieldset legend.