Gutenberg block editor support

[benlk]

[benlk]

A smattering of links:

• https://wptavern.com/wordpress-developers-learn-how-to-convert-shortcodes-to-gutenberg-blocks
  • https://gist.github.com/pento/cf38fd73ce0f13fcf0f0ae7d6c4b685d#file-php-block-js-L59 provides a PHP-first method for creating the block
• https://github.com/danielbachhuber/gutenberg-migration-guide doesn't apply here
• https://developer.wordpress.org/cli/commands/scaffold/block/
• https://wordpress.org/gutenberg/handbook/

[benlk]

Notes:

1. The shortcode continues to work, via the shortcode block.
2. We should port the shortcode to a block anyways.

[benlk]

Resolved with #34.