Replace Largo's custom post templating engine with WordPress' own page template functions

[benlk]

See all this code in https://github.com/INN/largo/blob/v0.6.4/inc/post-templates.php#L1-L115 ?

That predated the post template engine in WordPress core: https://developer.wordpress.org/themes/template-files-section/page-template-files/

Migration notes in https://make.wordpress.org/core/2016/11/03/post-type-templates-in-4-7/

Some questions that will require research:

• this template picker is normally only present on Pages; what is needed to make it available for Posts?
  • probably only the addition of the "Template Post Type: post, page" line in the file header
• what migration functionality is necessary?
• how will our template partials change?
• what needs to be done to allow site users to set a default template? Can we perform that change as part of the migration to the Customizer in Migrate Theme Options/Options Framework to Customizer controls #1438?