Display beta link on email signup pages

As we allow Users to signup for email alerts, we want to display to the
User that the service is currently in Beta.

This commit displays a beta banner if a flag is set in the details hash
in the content store.
Display link to email signup based on a flag

As we want the pages to exist without the link being displayed we need
to add a feature flag.

This commit displays the link based on a flag set in the details hash in
the content store item for the Finder.
Merge pull request #128 from alphagov/signup-beta-banner-and-signup-link-flag

Signup beta banner and signup link flag