Gutenberg support

There are two components to supporting Gutenberg:

1. Changing how Gutenberg posts are split, as using paragraph-tags approach breaks Gutenberg content.
2. Introducing a sidebar component to replace the legacy metabox.

Tasks:

• Detect if a post uses Gutenberg, introduce Gutenberg-capable splitting method and apply it
• Register meta
• Add wp-scripts
• Create sidebar component
• De-register metabox when on a Gutenberg post
• Fix issue where empty pages are added (it's probably the continue that should be a break.
• UNIT TESTS!!!

This should resolve #1 (closed), #5 (closed), #6 (closed), #7 (closed), #9 (closed), #10 (closed), and #11 (closed).