web: fix global navbar text-wrap

Created by: valerybugakov

Context

When all items in the global navbar are displayed, there's not enough space for them to render without text-wrap, which looks bad. This PR shrinks the horizontal margin between nav links by 4 pixels to fix the issue.

@sourcegraph/design, let me know if you see a better quick fix here without introducing a custom media query.

Screenshots

Before

After

Closes https://github.com/sourcegraph/sourcegraph/issues/23781.