Whilst editing a post it's possible to create a new template via the "Template" panel in the inspector. Doing so will create a $custom template that can subsequently be applied to any other post.

Sometimes it may be preferable to create a template just for the post currently being edited. We should offer the user the chance to choose their preference.

This can potentially reuse parts of the UI from the site editor where it is possible to create generic (e.g. page) or specific (e.g. page-$slug) templates.