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
For accessibility we should wrap this in a `fieldset` with a `legend` of something like:

> Which email alerts do you want to sign up for?

think of it as question/answer pairing.