Move link to providers page to the header

This link looked odd floating above the left column, and although we may want
to have admin navigation on the left we aren't sure what that would include
yet, so move this link to the header alongside the Platform admin link.

app/templates/admin_template.html

@@ -47,6 +47,9 @@
                 <li>
                   <a href="{{ url_for('main.platform_admin') }}">Platform admin</a>
                 </li>
+                <li>
+                  <a href="{{ url_for('main.view_providers') }}">Providers</a>
+                </li>
             {% endif %}
             <li>
               <a href="{{ url_for('main.sign_out')}}">Sign out</a>