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/subnavigation_component.rb

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