5.1.0: Compatibility fixes with WordPress 5.4

WordPress 5.4 / Gutenberg fixes

• Add support for block core/buttons that replaces old button block. Old content with core/button is still compatible.
• Remove custom styles from core/media-text in editor as new markup is very different from front-end
• Change responsive table markup and styles in core/table to exclude the figcaption from table styles
• Force correct max-width to core/separator also in editor
• Add featured image size in editor via filter
• Refactor the editor-gutenberg.scss according to new .block-editor class
• Skip some block types from getting default classname as it caused issues
• Minimize new whitespace from the bottom of the editor that pushes down meta boxes (.block-editor-writing-flow__click-redirect)
• Disable gradients

Other

• Remove duplicate _core-gallery.scss file