Prevent trashed customize_changeset posts from being erroneously mutated

[westonruter]

When revisions support is not added for customize_changeset posts, then when a changeset is transitioned (from auto-draft) to the publish status, then it will immediately be transitioned to trash so that it will be garbage collected. There are two problems with this right now in core:

1. If any of the JSON content has any strings containing HTML that kses doesn't like, it will get stripped when calling wp_trash_post() (if a privileged user is not logged in when published. e.g. during WP Cron). See #38715.
2. The post_name is mutated by appending __trashed. This is problematic when someone bookmarks a customizer session. If they return to the session and the changeset has been published or trashed, they should be shown a notice to that effect. With the suffix being added, this then fails for trashed changesets.

See _wp_customize_publish_changeset() for where wp_trash_post() is called.

[westonruter]

​https://github.com/xwp/wordpress-develop/pull/198

[westonruter]

In 39180:

Customize: Prevent post_content and post_name from being modified when trashing customize_changeset posts.

See #30937.
Fixes #38719.