Better scope CSS changes after #14407 to avoid plugins layout breakage

[afercia]

#14407 introduced some CSS changes that can break the UI provided by plugins.

Specifically the margins applied using the [data-block] selector, is breaking something in the Yoast SEO metabox and can potentially break other plugins. The Yoast SEO metabox uses some editable fields generated by Draft.js, which does use a data-block="true" attribute.

Broken layout:

Previous (and expected) layout:

Can this change be revisited please? This selector is too broad, not scoped with any specific Gutenberg class and, at the very least, should be restricted to only the editor content area.