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

https://www.pivotaltracker.com/story/show/81949808
https://www.pivotaltracker.com/story/show/80810718

This changes sub-topic pages to add a link to the email alerts sign up form and the latest feed. The links are hidden with a feature flag, which will be overridden at deploy-time to enable it in preview.
good catch. fixed