Theme starter content: Add support for featured images and page templates.

Featured image support means that attachments can now be imported. Media can be sideloaded from within theme or plugin directories. Like other posts, attachments are auto-drafts until customizer changes are published, and are not duplicated when they already exist in the customized state. Attachment IDs can be used for any number of purposes, much like post IDs. Twenty Seventeen now includes 3 images used as featured images to best showcase the multi-section homepage setup.

As featured image IDs are stored in post meta, it also made sense to add support for page templates. Twenty Seventeen does not include any such templates, but the functionality can be quite important for displaying themes to their best effect.

props westonruter, helen, flixos90.
fixes #38615.

Built from https://develop.svn.wordpress.org/trunk@39346


git-svn-id: https://core.svn.wordpress.org/trunk@39286 1a063a9b-81f0-0310-95a4-ce76da25c4cd