wp_editor calls non-existent filter callback on front end in multisite mode

[wonderboymusic]

When you are using wp_editor() on the front-end in Multisite mode, the editor will run code that applies upload_size_limit filter.

The default ms callback for 'upload_size_limit' is only accessible from the admin.

add_filter( 'upload_size_limit', 'upload_size_limit_filter' );

wp-includes/ms-functions.php is always loaded in multisite, so I moved the 4 necessary functions there from wp-admin/includes/ms.php

[nacin]

Okay.

[westi]

In 22497:

Multisite: Move all the functions which calculate upload space usage to wp-includes so they apply correctly with front-end editors.

Fixes #22357 props wonderboymusic.

[scribu]

Related: #22585