Bring the subtitle/subhead/dek meta box into Largo

[benlk]

https://github.com/INN/umbrella-publicsource/pull/37 is the most up-to-date version of this functionality, but it doesn't include support for Gutenberg.

Either in Largo or in a plugin:

• Bring in the metabox files
• Don't register our stuff if the child theme has its own versions of the functions
• Create a Gutenberg version of the box so that the page subtitle can be put in Gutenberg's document sidebar rather than in the metabox pile at the bottom of the editor. This would be one of:
  • a metadata block: https://developer.wordpress.org/block-editor/tutorials/metabox/meta-block-1-intro/
  • a control in the sidebar, which is apparently doable with add_meta_box and a context value of 'side' : https://stackoverflow.com/questions/51845266/how-to-add-new-panel-under-document-in-gutenberg

What I'm not sure about is whether this should be in Largo or in a plugin that can be updated separately from Largo, in response to Gutenberg changes.

[benlk]

If https://github.com/benhuson/wp-subtitle/issues/52 gets fixed so that the post metadata key can be filtered in all cases, we might want to consider:

• not moving https://github.com/INN/theme-calhealthreport/pull/19 into Largo
• instead, add to Largo's Theme Options:
  • a heading for post subtitles
  • a link saying that we recommend using https://wordpress.org/plugins/wp-subtitle/ to implement post subtitles
  • a checkbox that enables a filter on the name of the post metadata key to make it match Largo's historical option
  • template updates to pull the post metadata from the filtered post metadata key

[benlk]

@joshdarby I'm passing this ticket off to you; the maintainer updated it: https://github.com/benhuson/wp-subtitle/issues/52#issuecomment-909655392