Allow a super admin to demote themselves on a site to a role that does not...

Allow a super admin to demote themselves on a site to a role that does not contain the promote_users cap. props fonglh, fixes #18164.

git-svn-id: https://core.svn.wordpress.org/trunk@19024 1a063a9b-81f0-0310-95a4-ce76da25c4cd