Fix site navigation being somewhat jumbled at certain viewport widths

[strogonoff]

What's the problem this PR addresses?

Site navigation issues with bits of text overlapping each other and becoming unreadable, as well as awkward typography.

Screenshot before the fix:

How did you fix it?

• Allow horizontal scrolling of first top nav group
• Avoid awkward linebreaks
• Remove faux placeholder from search button that tries to act as a text input

Screenshot after the fix (note that the scrollbar is hidden unless scrolling is initiated or OS is set up to “always show” scrollbars):

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.

[arcanis]

Seems reasonable, thanks!