cc: Set island mode in `Main.tsx`'s constructor

`Main.tsx`'s `updateStatus()` was being called every 10 seconds,
which meant that `islandMode` was updated every 10 seconds.
If you were on the configuration page, and you refreshed the page
at the right moment (as soon as/just before `updateStatus()`
was being called), the config tabs showed were correct according to
the mode. If you refreshed at the wrong moment, you would get the
default tabs i.e. the tabs shown for the advanced mode (see
`ConfigurePage.js`'s `getSectionsOrder()`).

monkey/monkey_island/cc/ui/src/components/Main.tsx

@@ -70,6 +70,7 @@ class AppComponent extends AuthComponent {
       noAuthLoginAttempted: undefined
     };
     this.interval = undefined;
+    this.setMode();
   }
 
   updateStatus = () => {