Only add 'weekly' cron schedule interval if it does not already exist

git-svn-id: https://plugins.svn.wordpress.org/subscribe2/trunk@842061 b8457f37-d9ea-0310-8a92-e5e31aec5664
Add capability to add social media share buttons to HTML emails

git-svn-id: https://plugins.svn.wordpress.org/subscribe2/trunk@842063 b8457f37-d9ea-0310-8a92-e5e31aec5664