Add button to approve broadcast

Since new broadcasts will go into `pending-approval`, we now need a way
of approving them.

This commit adds a button to this page to start (or approve) the
broadcast. This button is wrapped in a bordered box, to emphasise that
it’s something consequential.

app/assets/stylesheets/components/banner.scss

@@ -24,6 +24,10 @@
     @include copy-19;
   }
 
+  .page-footer {
+    margin-bottom: govuk-spacing(1);
+  }
+
 }
 
 %banner-with-tick,