Better Update Notification

## What?
The update notification banner is good for knowing when new releases are available but the link isn't very useful, it just takes us to the graylog home page.

## Why?
Whenever I see the update notification I am curious what new features are available. It would be great if the link took the user to the changelog page (http://docs.graylog.org/en/3.2/pages/changelog.html) or to a blogpost or similar type of informative article regarding what is in the update.