Introduce new is_multi_author() template tag to make it easier for themes to...

Introduce new is_multi_author() template tag to make it easier for themes to have different behaviour when a site has more than one 
author. Fixes #14405 props filosofo.


git-svn-id: https://core.svn.wordpress.org/trunk@17901 1a063a9b-81f0-0310-95a4-ce76da25c4cd