Add additional promote_users checks. Show Add existing button only for...

Add additional promote_users checks.  Show Add existing button only for multisite.   Don't show bulk actions if user can't promote users.  Show add new user menu if user has create_users but not promote_users. Props nacin. fixes #16501 for trunk

git-svn-id: https://core.svn.wordpress.org/trunk@17431 1a063a9b-81f0-0310-95a4-ce76da25c4cd