Add beta banner to homepage

Because the homepage of the app is discoverable to the public, it makes
“gives the wrong impression” for it not to have a beta banner.

app/assets/stylesheets/app.scss

@@ -1,7 +1,7 @@
 // Extra CSS overlaying elements
 
 #global-header-bar {
-  background-color: $red;
+  background-color: $govuk-blue;
 }
 
 #global-header {
@@ -174,3 +174,8 @@ details summary {
   }
 
 }
+
+.phase-banner-beta {
+  border: 0;
+  margin-bottom: -$gutter + 2px;
+}

app/assets/stylesheets/main.scss

@@ -30,6 +30,7 @@ $path: '/static/images/';
 @import 'elements/layout';
 @import 'elements/lists';
 @import 'elements/panels';
+@import 'elements/phase-banner';
 @import 'elements/tables';