ISTWCMS-3830 Adding flex(css) to config form.

css/uw_dashboard.css

 .content-management-block-table > tbody > tr > td:first-child {
   width: 75%;
 }
+
+.flex-container > div {
+  display: flex;
+}
+
+.flex-container > div > fieldset {
+  flex-grow: 1;
+  flex-basis: 0;
+}

src/Plugin/Block/ContentManagementMenuBlock.php

@@ -158,6 +158,9 @@ class ContentManagementMenuBlock extends BlockBase implements ContainerFactoryPl
     $form['details'] = [
       '#type' => 'fieldset',
       '#title' => $this->t('Menus to display'),
+      '#attributes' => [
+        'class' => ['flex-container'],
+      ],
     ];
 
     foreach ($menu_first_level as $item_first_level) {