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/users/_statistics.html.erb

@@ -1,6 +1,6 @@
 <div>
   <h2>Statistics</h2>
-  <table width="100%" class="user-statistics">
+  <table width="100%" class="user-statistics table-sm">
     <tbody>
       <tr>
         <th>User ID</th>