Turn is_blog_user() into a wrapper around is_user_member_of_blog() and...

Turn is_blog_user() into a wrapper around is_user_member_of_blog() and deprecate.  Make user_id optional for is_user_member_of_blog(). Props SergeyBiryukov. fixes #16702

git-svn-id: https://core.svn.wordpress.org/trunk@19016 1a063a9b-81f0-0310-95a4-ce76da25c4cd