Updates to JSON API Post endpoint

Ads the following commits.
-  REST API: Adds current_user_can node to post response to pass
per-post capabilities
 REST API: don't check if a post is freshly pressed if
`is_post_freshly_pressed()` is not defined (e.g. on a Jetpack site)
fixes #1245
- REST API: POST Endpoint: Return featured_image for Jetpack Sites.
- REST: Add post title to parent post ref on post objects.