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/user_upgrades.scss

@@ -1,26 +1,6 @@
 div#c-user-upgrades {
   div#a-new {
-    margin: 0 auto;
-
-    * {
-      margin-bottom: 1em;
-    }
-
-    h1 {
-      text-align: center;
-    }
-
     table#feature-comparison {
-      width: 100%;
-
-      th {
-        font-weight: bold;
-      }
-
-      colgroup {
-        width: 10em;
-      }
-
       colgroup#basic {
         background-color: var(--user-upgrade-basic-background-color);
       }
@@ -32,12 +12,6 @@ div#c-user-upgrades {
       colgroup#platinum {
         background-color: var(--user-upgrade-platinum-background-color);
       }
-
-      td, th {
-        text-align: center;
-        vertical-align: top;
-        padding: 0.5em 0;
-      }
     }
   }
 }