Refactor Finders to pull from content_store

This commit contains the work needed to pull Finders from the
content_store and removes the ability for Finders to be retrieved from
the content-api.

Finders and email-alert-subs now pull their content_item and the
relevant tests have been updated to stub the content_store.
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 initialization 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 also refactors the presenter to use the standard language for using
content_items.
Merge pull request #97 from alphagov/display-email-signup-links

Display email signup links