Apply consistent spacing on the post visibility popover

[chrisvanpatten]

This is a small tweak to the padding/spacing on the post visibility popover.

Before:

After:

I also tweaked the class for the <fieldset> to be more consistent with the other BEM classes.

Note that @aduth had a question about the nested CSS classes on the original PR #9138 which I have not resolved here. It's a totally fair point, but should probably be part of a bigger discussion on CSS standards, as there are many cases of nested CSS rulesets (that probably don't need to be nested) around the Gutenberg code base.

[jasmussen]

Would it break the visuals if you replaces this with the new 4px $grid-size-small variable? I'm slowly trying to roll out the beginnings of a grid, and it'd be nice to use the new units wherever we use 4, 8 or 16px widths.

[jasmussen]

This is nicely harmonious looking. Nice pull requests. Visually I have no objections. I think the CSS looks cool too, but I also realize there are some intricacies that I haven't fully grokked, so okay to await a quick sanity check on that one. Thanks!

[chrisvanpatten]

Done! Thanks for the tip @jasmussen 👍

[aduth]

I also tweaked the class for the <fieldset> to be more consistent with the other BEM classes.

For clarity's sake, it's really a correction of an invalid class name moreso than it merely for consistency.

The editor-post-visibility class name would only be valid for the singular root element of PostVisibility. Since there is no root element (it returns an array), it's not valid to apply anywhere else.

https://github.com/WordPress/gutenberg/blob/master/docs/reference/coding-guidelines.md#naming