[skip ci] Use default button styles for .btn-secondary

app/assets/stylesheets/bootstrap_migration.scss

@@ -133,3 +133,8 @@ table {
     @extend .border-0;
   }
 }
+
+.btn-secondary {
+  // btn-default has been renamed btn-secondary; need to change all instances of btn-default
+  @include btn-default;
+}