Override CSS margins inherited from Gutenberg.

[afercia]

Summary

This PR can be summarized in the following changelog entry:

• Override CSS rules inherited from Gutenberg that break the Snippet Editor title and description

Test instructions

• install and activate the Gutenberg plugin, current version: 5.9.2
• on Yoast SEO trunk, edit a post, go to the metabox and open the snippet editor
• see the SEO title and description fields display some unexpected spacing, e.g.:

• switch to this branch and build the CSS
• refresh the page
• see the SEO title and description fields look as expected, e.g.:

Note: a PR has been submitted upstream to better scope the offending CSS selector, see WordPress/gutenberg#16207

Fixes #13145

[dariaknl]

Tested in this feature branch:

• fixed in metabox

• still not fixed in sidebar:

[afercia]

Latest commit fixes also the snippet editor fields within the modal dialog that opens from the sidebar.