Introduce partial to represent entire post part template

[westonruter]

When editing postmeta or some aspect of a post that does not have a specific partial, the entire post will need to be updated in the preview as opposed to just the specific partial representing the field modified. In all of the core themes, the posts have a content template part which is a perfect candidate for this. So when editing a post field or post meta that doesn't have a partial (or placement) in the preview, the fallback should be refreshing the entire post template part. This will require a new WP_Customize_Partial that is aware of the path to the template part, whether it is content, template-parts/content.