Add metabox and other functions for subhead

[benlk]

Changes

This pull request makes the following changes:

• adds a meta box for the subtitle
• provides a version of the solution for this problem in Largo (Bring the subtitle/subhead/dek meta box into Largo WPBuddy/largo#1730) that doesn't involve reliance on Largo's functions, because this is not a Largo child theme.
• tests the subtitle output on posts with $subtitle = get_post_custom_values( 'subtitle', get_the_ID() );

No styles are included here; I'm waiting until #21 and the firmed-up subtitle styles from that.

Why

For #14, as a version of WPBuddy/largo#1730 that doesn't depend upon Largo

Testing/Questions

Features that this PR affects:

• Editor for posts, in Gutenberg and Classic
• Single post template

Questions that need to be answered before merging:

• Is this PR targeting the correct branch in this repository?
• Should we try to allow some HTML in the subtitle?

Steps to test this PR:

1. edit a post's subtitle, click "Update" or "Save".

[benlk]

Nonce check is currently not passing in the Classic editor.

[joshdarby]

Works as expected! 👍