Make left-hand navigation into a 1/4 width column

Since we’ve simplified the left hand nav so much it doesn’t need 1/3 of
the width of the page. By making it smaller we have more space to show
previews of email and letter templates.

app/templates/withnav_template.html

@@ -13,10 +13,10 @@
       <a href="{{ url_for('main.choose_service') }}" class="navigation-service-switch">Switch service</a>
     </div>
     <div class="grid-row">
-      <div class="column-one-third">
+      <div class="column-one-quarter">
         {% include "main_nav.html" %}
       </div>
-      <main role="main" class="column-two-thirds column-main">
+      <main role="main" class="column-three-quarters column-main">
         {% include 'flash_messages.html' %}
         {% block maincolumn_content %}{% endblock %}
       </main>