Consolidate larg network criteria into wp_is_large_network(). Allow plugins...

Consolidate larg network criteria into wp_is_large_network().  Allow plugins to change this criteria via filter. Props PeteMall. fixes #18464

git-svn-id: https://core.svn.wordpress.org/trunk@18871 1a063a9b-81f0-0310-95a4-ce76da25c4cd