css: refactor CSS to use Tailwind-style utility classes.

Refactor CSS to use standard Tailwind-style utility classes instead of
ad-hoc rules. This eliminates a lot of single-purpose rules for specific
UI elements and standardizes margins to be more consistent throughout
the site.

Utility classes are defined manually on an as-needed basis instead of
importing Tailwind as a whole. Naming conventions mostly follow
Tailwind's conventions, otherwise they follow Bootstrap.

* https://tailwindcss.com/docs/
* https://getbootstrap.com/docs/5.0/utilities/spacing/

app/javascript/src/styles/specific/post_versions.scss

@@ -7,10 +7,6 @@ body.c-post-versions.a-index {
     min-width: 2em;
   }
 
-  .advanced-search-link {
-    margin: 0 1em;
-  }
-
   ins.diff-obsolete a {
     color: var(--diff-list-obsolete-added-color);
     font-weight: bold;