Remove <h2> tag around service name in nav

The heading structure of most pages is incorrect (`<h2>` followed by
`<h1>`). The `<h1>` indicates the main purpose of the page, the service
name (currently the first `<h2>`) doesn't need to be a heading.

app/templates/withnav_template.html

@@ -7,9 +7,9 @@
 {% block fullwidth_content %}
   <div id="content">
     <div class="navigation-service">
-      <h2 class="navigation-service-name">
+      <div class="navigation-service-name">
         {{ current_service.name }}
-      </h2>
+      </div>
       <a href="{{ url_for('main.choose_service') }}" class="navigation-service-switch">Switch service</a>
     </div>
     <div class="grid-row">