Stick batch table toolbar to the top (#20442)

Fixes #20441

app/javascript/styles/mastodon/tables.scss

@@ -178,6 +178,9 @@ a.table-action-link {
   }
 
   &__toolbar {
+    position: sticky;
+    top: 0;
+    z-index: 1;
     border: 1px solid darken($ui-base-color, 8%);
     background: $ui-base-color;
     border-radius: 4px 0 0;