Fixes #6409 - Changes the reader detail to show only excerpts for Jetpack posts that have them. Note the design of the "Visit Site Name for more" footer was approved by Derek (reader squad designer and lead).

To test:

• View this post in Calypso reader https://wordpress.com/read/blogs/67272686/posts/7751
• Like the post
• Open the Android reader to "Posts I Like" and view the post detail for the above post