BlogGen Connector v1.1.15

Featured image publish hardening:
- Accepts all Hub featured image keys: featured_media, featured_media_id, featured_image_connector_id and featured_image_id.
- Forces and verifies the WordPress _thumbnail_id meta when the image has been imported.
- Returns featured image confirmation data to the Hub so the Hub can warn if the client site did not confirm attachment.
- Keeps the v1.1.14 base64 embedded image fallback for sites that block remote image sideloads from the Hub.
