Reenable display of js-enabled button elements

This was added in a refactor commit (7dbeca7) and had the effect of hiding the
sign up subscription button. Not sure if that's whats intended, but it should
definitely be there now.
Add checkboxes on email signup.

This depends on the correct choice data format being in the content store so
shouldn't be deployed until after the associated change in finder-api.
Improve styling of checkboxes to match govuk elements.
Update finder frontend email signup to match actual prototype.
Remove quiet helptext

This was not on the prototype and nobody in user research had difficulty understanding that they should enter an address.
Remove full stop

I don't think 'Select the email alerts you need' is a sentence so, so removing this.
Removing unused class
Give the legend some bottom padding
Use .checkbox not a pseudo selector

Pseudo selectors don't work in IE6 so this page was pretty borked. Using a checkbox class instead.
Add color for legend which is blue(?) otherwise in ie6/7
Get rid of display:block

The block-label class is getting it blockness as a consequence of being floated left, so we don't need to explicitly state it with display:block too
Use colours from the frontend toolkit instead of hex
Use $gutter values from frontend toolkit
Add in mobile styles for form

- Move max-width restriction into media query so mobile has a full width form
- Make checkbox labels full width on mobile
remove newline
Bump govuk_frontend_toolkit gem
Add JS to make checkboxes work