Editor: Fix incorrect usage of ID attributes on custom fields.

Repeated containers used for custom fields have duplicate ID attributes. Duplicate IDs are incorrect HTML, and will also cause unexpected results when trying to manipulate using JS. Duplicate IDs are changed to matching classes; CSS & JS updated to match. 

Props jankimoradiya, audrasjb, donmhico, afercia.
Fixes #46964.

Built from https://develop.svn.wordpress.org/trunk@47222


git-svn-id: https://core.svn.wordpress.org/trunk@47022 1a063a9b-81f0-0310-95a4-ce76da25c4cd