Customize: Allow post/page stubs to be edited in WP Admin as "customization...

Customize: Allow post/page stubs to be edited in WP Admin as "customization drafts" when changeset is saved as draft or scheduled.

* Update stubs to have draft status when changeset is saved as draft, instead of preventing auto-draft garbage collection by giving them a far-future `post_date`.
* Show notice in publish metabox when editing a customization draft indicating that it will be published automatically with its changeset; a link to Customizer is included.
* Include a new "Customization Draft" display post state in the post list table.
* Disconnect stubs from their changesets when they are updated with a status other than "Draft".
* Trash customization drafts when their related changeset is trashed or deleted.
* Add a `_customize_changeset_uuid` postmeta to stubs to link them with their associated changeset.
* Include `customize_changeset_uuid` as context when requesting to insert a new auto-draft.

Props westonruter, melchoyce.
See #39896, #39752, #34923.
Fixes #42220.

Built from https://develop.svn.wordpress.org/trunk@41887


git-svn-id: https://core.svn.wordpress.org/trunk@41721 1a063a9b-81f0-0310-95a4-ce76da25c4cd