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/pools.scss

@@ -1,5 +1,3 @@
-@import "../base/000_vars.scss";
-
 a.pool-category-series, .pool-category-series a {
   color: var(--series-pool-color);
 
@@ -16,51 +14,18 @@ a.pool-category-collection, .pool-category-collection a {
   }
 }
 
-div#add-to-pool-dialog {
-  font-size: 0.8em;
-
-  form {
-    margin-bottom: 1em;
-  }
-
-  li {
-    margin-left: 1em;
-    cursor: pointer;
-  }
-}
-
-div#c-pools {
-  textarea {
-    height: 10em;
-  }
-
-  div#a-show {
-    div#description {
-      margin-bottom: 1em;
-    }
-  }
-}
-
 div#c-pool-orders, div#c-favorite-group-orders {
   div#a-edit {
-    ul.ui-sortable {
-      list-style-type: none;
+    ul.ui-sortable li {
+      display: inline-block;
+      min-width: 150px;
 
-      li {
-        cursor: pointer;
-        margin-right: 20px;
-        margin-bottom: 20px;
-        min-width: 150px;
-        min-height: 150px;
-        padding: 10px;
-        display: inline-block;
+      &, .post-preview a {
+        cursor: move;
       }
 
-      li.ui-state-default {
-        background: none;
-      }
-
-      li.ui-state-placeholder {
+      &.ui-state-placeholder {
+        border: none;
         background: none;
       }
     }