REST API: Include permalink_template/generated_slug for Posts

In order for clients to present permalink previews, the REST API must share the computed results of get_sample_permalink(). These two values are now exposed as permalink_template and generated_slug for public, viewable post types, but only for context=edit.

Merges [43720] to trunk.

Props danielbachhuber, rahulsprajapati.
Fixes #45017.

Built from https://develop.svn.wordpress.org/trunk@43980


git-svn-id: https://core.svn.wordpress.org/trunk@43812 1a063a9b-81f0-0310-95a4-ce76da25c4cd