Add a loading indicator component

This will let us design a page which tells the user they need to wait.

app/assets/stylesheets/main.scss

@@ -66,6 +66,7 @@ $path: '/static/images/';
 @import 'components/preview-pane';
 @import 'components/task-list';
 @import 'components/vendor/govuk-back-link';
+@import 'components/loading-indicator';
 
 @import 'views/dashboard';
 @import 'views/users';