Add a link to sign up to email alerts

This changes the sub-topic view to include a link to the email signup
form.
Use a feature flag to hide the email signup link

This adds a feature flag to restrict the display of the email signup
link in production environments.

We'll override this on preview to display the link by overriding this
initializer in the deploy scripts.
New layout for email alerts and latest feed link

This restyles the previously added email alerts link based on feedback
and suggestions from @futurefabric.

I've also added a link to the latest changes feed here.
Merge pull request #61 from alphagov/add-email-signup-link

Add a link to the latest feed and email alert sign up form