Tidy up navigation code

- use SASS variables for colours and spacing
- make navigation an include not a macro (because it doesn’t take any
  parameters)

app/templates/withnav_template.html

@@ -4,7 +4,7 @@
 {% block fullwidth_content %}
 	<div class="grid-row">
 	  <div class="column-one-quarter">
-	    {{ main_nav() }}
+	    {% include "main_nav.html" %}
 	  </div>
 	  <div class="column-three-quarters">
 	    {% block maincolumn_content %}{% endblock %}