Fix and simplify is_using_gutenberg()

[helen]

Description of the Change

has_blocks() does a get_post() if you don't pass a string, which doesn't work for external posts (which presumably is what you're pulling). Better to pass a string.

Also simplifies the rest of the function down to just reproducing the check from has_blocks() if you're not in WP 5.0+.

Alternate Designs

Considered leaving the rest of the function alone but thought it best to just do it all at once.

Benefits

Far simplified codebase, no more fatal error, no more false positives.

Possible Drawbacks

Not seeing any.

Verification Process

Testing pulling and pushing posts written using block editor, both in network and external connections.

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Applicable Issues

Don't have a companion issue open but you can see here http://s.hyhs.me/7bfef432f30c