Fix RTL display in Optional grouping on Permalinks Settings page

[sabernhardt]

splitting from #47755 (props rachid84, johnbillion, SergeyBiryukov, costdev, joedolson, afercia, ryokuhi, oglekler)

For the Category/Tag Base fields, subdirectory network installations can show a /blog prefix before the input field. In RTL languages, the slash is on the wrong side. Also, the example URL can wrap awkwardly to the next line. In any language, the text is stacked above the field with little space between them on smaller screens.

1. Create a multisite network with subdirectories.
2. Open the main site.
3. Visit Settings -> Permalinks.
4. Scroll down to the Optional section.

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).

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

[sabernhardt]

Screenshots from ​PR 2955

LTR with patch

​820 pixels wide, in English

​600 pixels wide, in English

RTL with patch

​820 pixels wide, in Arabic

​600 pixels wide, in Arabic