Override Gutenberg styles breaking the snippet editor

[afercia]

In WordPress/gutenberg#14407 Gutenberg introduced a CSS selector [data-block] used to control margins of the blocks in the editor.

Our Snippet Editor uses Draft.js which in turn uses some data-block attributes in its rendered HTML so the Gutenberg CSS rules are inherited and break the layout of the snippet editor title and description.

See WordPress/gutenberg#15465

A PR has been submitted upstream in the Gutenberg repo. To be 100% sure to avoid the breakage, we should override the Gutenberg CSS rule anyways.