Updates to the 'Log out everywhere' implementation.
 * Include a message and a disabled button when you're only logged in at one location.
 * Avoid leaking the session token in HTML.
 * Simplify, simplify, simplify.

Merges [30888] to the 4.1 branch.

Fixes #30264.

Built from https://develop.svn.wordpress.org/branches/4.1@30895


git-svn-id: http://core.svn.wordpress.org/branches/4.1@30885 1a063a9b-81f0-0310-95a4-ce76da25c4cd