Customize: Remove iframe-specific behaviors from customize preview when...

Customize: Remove iframe-specific behaviors from customize preview when previewing on frontend and not contained inside iframe.

* Strip out `customize_messenger_channel` from preview window URL when not contained in iframe.
* Allow interacting with unpreviewable links and forms when previewing customized state on frontend.

See #30937.
Fixes #38867.

Built from https://develop.svn.wordpress.org/trunk@39332


git-svn-id: https://core.svn.wordpress.org/trunk@39272 1a063a9b-81f0-0310-95a4-ce76da25c4cd