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/views/wiki_page_versions/_listing.html.erb

@@ -34,7 +34,7 @@
     <% end %>
 
     <% if listing_type(:wiki_page_id, member_check: false, types: [:page, :global]) == :page %>
-      <%= submit_tag "Diff" %>
+      <%= submit_tag "Diff", class: "mt-4" %>
     <% end %>
   <% end %>
 </div>