Jetpack: Featured Content: Lots of updates compliments of @obenland

Featured Content: Bring back improvements from Twenty Fourteen development.
 * Move settings to the Customizer.
 * Provide possibilities to set a default tag and a fallback if there are no featured posts with a certain tag.
 * Add braces to one-line conditionals.
 * Specify all supported post types rather than just additional ones.

Remove quantity setting from the options array.
 * The quantity of posts in the Featured Content area can be set by the amount of posts tagged with the featured tag, up to the the amount specified by the theme with $max_posts.

Update section description to inform users about how many posts the current theme supports.



git-svn-id: https://plugins.svn.wordpress.org/jetpack/trunk@815062 b8457f37-d9ea-0310-8a92-e5e31aec5664