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/explore/posts/_nav_links.html.erb

@@ -1,7 +1,7 @@
 <%# path, date, scale, %>
 
-<p class="date-nav-links">
+<div class="date-nav-links text-center md:space-x-8">
   <%= render "explore/posts/nav_links_for_scale", path: path, date: date, selected_scale: scale, scale: "day" %>
   <%= render "explore/posts/nav_links_for_scale", path: path, date: date, selected_scale: scale, scale: "week" %>
   <%= render "explore/posts/nav_links_for_scale", path: path, date: date, selected_scale: scale, scale: "month" %>
-</p>
+</div>