Jetpack: Featured Content: Prevent Featured_Content class from loading on plugin list page

Prevents the Featured_Content class from loading on the plugin list page. This allows other
plugins to load their own Featured_Content class without requiring an explicit check to
determine if Jetpack's Featured_Content class is loaded. Also ensure that class will be
the Featured_Content class that is used and Jetpack's own Featured_Content class will
automagically disable itself.
Merge branch 'master' of github.com:Automattic/jetpack