Prevent loading screen from overlaying the app bar

Refs #4417

This change is functional copy of 3893469 that solves the problem in a way
that doesn't depend on the height of the app bar since the app bar isn't a
fixed height in the smaller views.

ui/app/app.js

@@ -152,12 +152,14 @@ class App extends Component {
 
         h(AccountMenu),
 
-        (isLoading || isLoadingNetwork) && h(Loading, {
-          loadingMessage: loadMessage,
-        }),
+        h('div.main-container', [
+          (isLoading || isLoadingNetwork) && h(Loading, {
+            loadingMessage: loadMessage,
+          }),
 
-        // content
-        this.renderRoutes(),
+          // content
+          this.renderRoutes(),
+        ]),
       ])
     )
   }

ui/app/components/page-container/index.scss

@@ -182,5 +182,7 @@
     max-height: 82vh;
     min-height: 570px;
     flex: 0 0 auto;
+    margin-right: auto;
+    margin-left: auto;
   }
 }

ui/app/css/itcss/components/loading-overlay.scss

@@ -1,6 +1,6 @@
 .loading-overlay {
   left: 0;
-  z-index: 50;
+  z-index: 256;
   position: absolute;
   flex-direction: column;
   display: flex;