'Add Slideshow' UI fails with Gutenberg post editor.

[dg4220]

With the Gutenberg block editor the 'wp-content-editor-tools' area is gone. This is the area where the "Add Media" button as well as the 'Add Slideshow" and "Add Form" (for the Gravity Forms plugin) buttons were located on the post/page edit screen.

Slideshows can still be included using the 'Shortcode' Block. Ultimately we need to create a Custom Slideshow Block so users can add slideshows with gutenberg's Add Block UI.