Ensure 's2_authors' keys exist on all sites and for all Registered Users as per illmasterc’s bug report

git-svn-id: https://plugins.svn.wordpress.org/subscribe2/trunk@860824 b8457f37-d9ea-0310-8a92-e5e31aec5664
Ensure 's2_authors' keys exist on all sites and for all Registered Users as per illmasterc’s bug report

git-svn-id: https://plugins.svn.wordpress.org/subscribe2/trunk@860826 b8457f37-d9ea-0310-8a92-e5e31aec5664
Introduce 's2_mu_blog_list' filter to allow filter of blog list for WordPress Multisite installs - props careb

git-svn-id: https://plugins.svn.wordpress.org/subscribe2/trunk@860827 b8457f37-d9ea-0310-8a92-e5e31aec5664