Move convert_to_screen() back to template.php to avoid fatal errors with...

Move convert_to_screen() back to template.php to avoid fatal errors with plugins and themes that direct include template.php into the front end. Flag this bad behavior with _doing_it_wrong(). Props nacin. fixes #19342

git-svn-id: https://core.svn.wordpress.org/trunk@19428 1a063a9b-81f0-0310-95a4-ce76da25c4cd