Resurrect get_blog_list() and get_most_active_blogs() but leave deprecated....

Resurrect get_blog_list() and get_most_active_blogs() but leave deprecated. Props ocean90. fixes #13773

git-svn-id: https://core.svn.wordpress.org/trunk@15170 1a063a9b-81f0-0310-95a4-ce76da25c4cd