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';
 
 

app/templates/views/signedout.html

@@ -9,6 +9,13 @@
 
 {% block maincolumn_content %}
 
+  <div class="phase-banner-beta">
+    <p>
+      <strong class="phase-tag">BETA</strong>
+      <span>This is a new service – your <a href="{{ url_for('main.feedback') }}">feedback</a> will help us to improve it.</span>
+    </p>
+  </div>
+
   {% call banner_wrapper(type='intro') %}
     <h1 class="heading-medium">
       GOV.UK Notify makes it easy to send text messages and emails to