Add placeholder loader for Track Panel (#577)

src/ensembl/stories/sidebar-loader/SidebarLoader.stories.scss

+@import 'src/styles/common';
+
+.fullPageWrapper {
+  border: 1px solid $light-grey;
+  padding: 20px;
+  height: 500px;
+  width: 450px;
+}

src/ensembl/stories/sidebar-loader/SidebarLoader.stories.tsx

+/**
+ * See the NOTICE file distributed with this work for additional information
+ * regarding copyright ownership.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+import React from 'react';
+
+import { SidebarLoader } from 'src/shared/components/loader';
+
+import styles from './SidebarLoader.stories.scss';
+
+export default {
+  title: 'Components/Shared Components/SidebarLoader'
+};
+
+export const FullPageLoader = () => (
+  <div className={styles.fullPageWrapper}>
+    <SidebarLoader />
+  </div>
+);
+
+FullPageLoader.storyName = 'full-page';