Add restricted mode warning

For the hack day, we should only let developers use the platform in restricted
mode. This commit adds a banner telling them this.

Can’t get the app running locally, so fingers crossed it actually looks how
I imagine it’s going to look…

app/assets/stylesheets/components/banner.scss

@@ -28,6 +28,12 @@
 
 }
 
+.banner-info {
+  @extend .banner;
+  background: $govuk-blue;
+  color: $white;
+}
+
 .banner-dangerous {
 
   @extend .banner;