Refactor Official sharing button to respect new guidelines

https://vk.com/pages?oid=-17680044&p=Sharing_External_Pages

Add new function to get excerpt for Official sharing button
Add some missing escaping, props @jkudish



git-svn-id: https://plugins.svn.wordpress.org/vk-sharing-jetpack/trunk@950110 b8457f37-d9ea-0310-8a92-e5e31aec5664