Support for external images

[WPprodigy]

Issue Overview

To come across this issue, I copy/pasted a blog article from my live site onto a local testing site. The result of this is some console errors for each image I had originally embedded on the live site:

This is happening because the image blocks store an ID like this <!-- wp:image {"id":909} -->, and said image doesn't actually exist on my local site. So this REST API call fails essentially:

gutenberg/blocks/library/image/block.js

Line 298 in 003d533

image: `/wp/v2/media/${ id }`,

The block settings adjust correctly and don't display the option to change the image's size, but instead of correcting this error for future page-loads, Gutenberg just leaves it as-is and continues to make api requests that fail.

I'm not really sure what the best solution here is, but I'm thinking that support for linking to external images would be the cleanest path. And when an image ID is found to not exist, it could strip away the ID property in the comment object.

[mtias]

Related #1536

[mtias]

Closing in favor of #1536, #9264, and #9301.