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/components/layout/footer_component.rb

@@ -0,0 +1,2 @@
+class Layout::FooterComponent < ApplicationComponent
+end

app/components/layout/subnavigation_component.rb

@@ -0,0 +1,3 @@
+class Layout::SubnavigationComponent < ApplicationComponent
+  delegate :content_block, :layout_menu_link_to, to: :helpers
+end

app/views/layouts/_header.html.erb

@@ -30,7 +30,7 @@
 
         <div class="show-for-small-only">
           <div class="subnavigation subnavigation-with-top-links">
-            <%= render "shared/subnavigation" %>
+            <%= render Layout::SubnavigationComponent.new %>
             <%= render "shared/top_links" %>
           </div>
         </div>
@@ -40,7 +40,7 @@
 
   <div id="navigation_bar" class="subnavigation">
     <div class="hide-for-small-only">
-      <%= render "shared/subnavigation" %>
+      <%= render Layout::SubnavigationComponent.new %>
     </div>
 
     <%= yield :header_addon %>

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>

app/views/layouts/devise.html.erb

@@ -32,7 +32,7 @@
     </div>
 
     <div class="footer">
-      <%= render "layouts/footer" %>
+      <%= render Layout::FooterComponent.new %>
     </div>
   </body>
 </html>