Customize: Do not allow changesets to be deleted when someone is editing them.

This makes the behavior consistent with that of locked posts, which can't be deleted via the list tables when another user is editing them.

Props dlh.
Fixes #50501.
Built from https://develop.svn.wordpress.org/trunk@48211


git-svn-id: https://core.svn.wordpress.org/trunk@47980 1a063a9b-81f0-0310-95a4-ce76da25c4cd