Use throbber in Standalone layout; center loading messages and content

src/standalone/layout.jsx

@@ -1,3 +1,5 @@
+
+
 import React from "react"
 import PropTypes from "prop-types"
 
@@ -32,13 +34,17 @@ export default class StandaloneLayout extends React.Component {
         { Topbar ? <Topbar /> : null }
         { loadingStatus === "loading" &&
           <div className="info">
-            <h4 className="title">Loading...</h4>
+            <div className="loading-container">
+              <div className="loading"></div>
+            </div>
           </div>
         }
         { loadingStatus === "failed" &&
           <div className="info">
+            <div className="loading-container">
               <h4 className="title">Failed to load spec.</h4>
             </div>
+          </div>
         }
         { loadingStatus === "failedConfig" &&
           <div className="info" style={{ maxWidth: "880px", marginLeft: "auto", marginRight: "auto", textAlign: "center" }}>

src/style/_layout.scss

@@ -653,6 +653,10 @@
 .loading-container
 {
     padding: 40px 0 60px;
+    min-height: 1px;
+    display: flex;
+    justify-content: center;
+
     .loading
     {
         position: relative;