4.0: Let's just hope this doesn't break.

Merges r101815-wpcom.
REST API: Add 'plugin' type to cast_and_filter_item.

Used to cast and filter the result of the list plugins endpoint.
+ Add 'safehtml' type applies wp_kses to the response value. Used to sanitize the plugin description.

Merges r101961-wpcom.
REST API: add post thumbnail image object as part of the post response

see https://[private link]

Merges r101978-wpcom.
REST API: unhook publicize from its usual `save_post` hook and rehook it on the `rest_api_inserted_post` hook so that all the meta can get set properly before publicize runs. Otherwise, a risk for a race condition occurs, in which case for example a skipped service might not get skipped properly

see https://[private link]

Merges r101872-wpcom.
REST API: prevent Path from publicizing, it's kind of redudant

Merges r101937-wpcom.
Customizer: add event namespacing to Widget Conditions and WP.com specific JS. See #6714.

Merges r101700-wpcom.