Form Validation: Post editor (block editor)

[sc0ttkclark]

We need to prevent the block editor saving process when there are fields in an invalid state (required, bad format, etc).

• Stop saving of a new post
• Stop saving of a draft
• Stop updating of a published post

Stop saving/updating means:

• Do not save the post
• Do not save the metaboxes

Bonus points:

• Show the error in the publish flow (when publishing a new post)
• Show a notice on the screen about errors being highlighted in the form below (maybe link to the first metabox?)

[sc0ttkclark]

@alexstandiford is going to have a go at a proof of concept to give us a head start here.