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