Display email signup links

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.

Screenshot:
![screen shot 2014-10-03 at 14 44 51](https://cloud.githubusercontent.com/assets/449004/4721229/4832094c-5937-11e4-8dd6-bfa3c504eea1.png)

This also refactors the presenter to use the standard language for using content_items.
Could `email_alert_signup` be extracted to another class method here?

```ruby
def self.finder_email_signup(content_item)
  if blabla
end
```

It also feels like this would be better as the responsibility of the `content_item` to provide its finder_email_signup.