Fix RTL display in Optional grouping on Permalinks Settings page

[sabernhardt]

In the main site of subdirectory network installations, the blog prefix does not display correctly in RTL.

LTR (at 820 pixels wide and at 600):

In RTL, the slash displays on the wrong side of "blog" and the prefix appears on the wrong side of the input field. Plus, the example URL can wrap awkwardly to the next line.

This wraps the prefix and field in a span with the code class and adds CSS to keep the prefix next to the field on smaller screens. In the paragraph, code elements are set to inline-block to keep them on one line (or wrap better).

I also wrapped the Optional group in a fieldset and added aria-describedby to use the paragraph as context to fill out this group of fields. This required a few CSS adjustments for margin and padding.

Trac ticket: https://core.trac.wordpress.org/ticket/58722

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[sabernhardt]

Updated screenshots after removing the fieldset and legend

LTR with patch

RTL with patch

[sabernhardt]

This has its own separate ticket 58722 now.

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[github-actions]

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[sabernhardt]

I added nowrap to avoid breaking the prefix on two lines in Firefox. Also, the condition did not need to use the empty() function when $blog_prefix can only be either /blog or an empty string.

Having the prefix on the left can make the input field a bit smaller. However, it should be enough, even on 320-pixel screens (where the field is about 248 pixels wide, and can show 24 characters before scrolling).