Add secondary link pattern to page footer

This commit brings back the ‘link under the green button’ bit of the page footer
component.

Previous it had been changed to be a grey button. But there are use cases for
both, maybe even simultaneously.

app/templates/views/styleguide.html

@@ -117,6 +117,10 @@
     button_text='Send', back_link='http://example.com', back_link_text="Back to dashboard"
   ) }}
 
+  {{ page_footer(
+    button_text='Sign in', secondary_link='http://example.com', secondary_link_text="I’ve forgotten my password"
+  ) }}
+
   <h2 class="heading-large">SMS message</h2>
 
   <p>Used to show, preview or choose an SMS message.</p>