Email signup links for content stored Finders

In order to for users to signup for alerts from Finders, we need to display a link to the signup page. This commit parses the response from the content store and pulls out the finder_email_signup field in the Finder initialisation if it's present. The nil check happens in the view, so we assign it as a nil value if the field isn't in the response.

This commit also adds the mail icon (along with a retina version) and the CSS required to display this.

This relies on https://github.com/alphagov/finder-api/pull/64 and https://github.com/alphagov/finder-frontend/pull/95 being merged first.
Can this be expressed in `$gutter` variables rather than `em` so it matches other measurements in this file.