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/posts/index.html.erb

@@ -16,7 +16,7 @@
 <% end %>
 
 <% content_for(:content) do %>
-  <menu id="post-sections">
+  <menu id="post-sections" class="mb-4">
     <li class="active"><a href="#" id="show-posts-link">Posts</a></li>
 
     <% if @post_set.artist.present? %>