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/users.scss

@@ -5,24 +5,10 @@ div#c-users {
     }
 
     table.user-statistics {
-      tr {
-        height: 1.75em;
-      }
-
       th {
         width: 15%;
         text-align: right;
         padding-right: 1em;
-        vertical-align: top;
-      }
-
-      td {
-        width: 85%;
-        vertical-align: top;
-      }
-
-      p {
-        margin-bottom: 0.5em;
       }
 
       .user-verified-email-icon {
@@ -36,14 +22,6 @@ div#c-users {
   }
 
   div#a-edit {
-    h1 {
-      margin: 0.5em 0;
-    }
-
-    h2 {
-      margin: 0.5em 0;
-    }
-
     div.input {
       margin-bottom: 2em;
     }