Move navigation and footer partials to components

This way it'll be easier to decide when they should be rendered.

In order to be consistent, we're using the `Layout` module for both
components; previously, the navigation partial was in the `shared`
folder while the footer partial was in the `layout` folder, which IMHO
didn't make much sense.

app/views/layouts/application.html.erb

@@ -43,7 +43,7 @@
       <%= yield %>
     </div>
     <div class="footer">
-      <%= render "layouts/footer" %>
+      <%= render Layout::FooterComponent.new %>
     </div>
   </body>
 </html>