Add a tour for users new to broadcast services

This is an initial, prototype-quality attempt at introducing some kind
of tour for users new to broadcasting. A lot of the users we’re speaking
to don’t have a good concept of what broadcasting means, which is
causing usability problems down the line.

We did a similar thing in the early days of Notify to explain the
concept of message templates and personalisation.

app/assets/stylesheets/components/banner.scss

@@ -78,6 +78,11 @@
   box-shadow: inset 0 -1em 1.6em 0 rgba(0, 0, 0, 0.05);
   border: 0;
 
+  &-no-fixed-height {
+    height: auto;
+    min-height: 425px;
+  }
+
   .heading-medium {
     @include core-24;
   }
@@ -98,6 +103,18 @@
 
   }
 
+  ul {
+    @include core-24;
+    color: $white;
+    margin-bottom: govuk-spacing(5);
+  }
+
+  li {
+    margin-bottom: govuk-spacing(3);
+    list-style-type: disc;
+    margin-left: govuk-spacing(6);
+  }
+
   a {
 
     font-weight: bold;
@@ -123,6 +140,10 @@
     }
 
   }
+  img {
+    width: 100%;
+    height: auto;
+  }
 
   .greyed-out-step {
     opacity: 0.6;