fix issue with spinner icon & longrunning active state

- using padding meant the icon would visually spin 'wobbly'
- button content is no longer centre aligned so button longrunning active state should not be either
- fixes #8615

client/scss/components/_button.scss

@@ -237,7 +237,6 @@
 
     &-active {
       display: inline-flex;
-      justify-content: center;
       align-items: center;
     }
 

wagtail/admin/static_src/wagtailadmin/scss/layouts/page-editor.scss

@@ -475,7 +475,7 @@ footer .preview {
     @apply w-leading-none w-inline-flex w-items-center;
 
     .icon {
-      @apply w-pr-2;
+      margin-inline-end: theme('spacing.2');
     }
   }
 }