Make is_upload_space_available() available for front end page loads. Fixes...

Make is_upload_space_available() available for front end page loads. Fixes fatal error when calling wp_editor() from the front end.

Props scribu, itworx
fixes #22585


git-svn-id: https://core.svn.wordpress.org/trunk@22840 1a063a9b-81f0-0310-95a4-ce76da25c4cd