Replace spin.js with a CSS-only loading spinner (#2764)

* Create CSS only loading indicator

* Core mods to fix Loading Indicator usage

* Remove extra whitespace

* Attrs interface extends ComponentAttrs and is exported

* Add doc block about custom styling

less/common/Checkbox.less

@@ -15,13 +15,9 @@
     float: left;
     margin-left: -65px;
     margin-top: -4px;
-
-    .LoadingIndicator {
-      display: inline-block;
-      margin-left: 20px;
-    }
   }
 }
+
 .Checkbox--switch .Checkbox-display {
   width: 50px;
   height: 28px;
@@ -31,8 +27,28 @@
   background: @control-bg;
   .transition(background-color 0.2s);
 
+  .LoadingIndicator {
+    --size: 22px !important;
+
+    &-container {
+      height: 22px;
+    }
+  }
+
   .on& {
     background: #58a400;
+
+    .LoadingIndicator-container {
+      // Show loading indicator over the switch button
+      justify-content: flex-end;
+    }
+  }
+
+  .off& {
+    .LoadingIndicator-container {
+      // Show loading indicator over the switch button
+      justify-content: flex-start;
+    }
   }
 
   &:before {