Use relative URLs where possible

In general, we always use relative URLs (using `_path`), but sometimes
we were accidentally using absolute URLs (using `_url`). It's been
reported i might cause some isuses if accepting both HTTP and HTTPS
connections, although we've never seen the case.

In any case, this change makes the code more consistent and makes the
generated HTML cleaner.

app/controllers/admin/widget/cards_controller.rb

@@ -60,7 +60,7 @@ class Admin::Widget::CardsController < Admin::BaseController
       if @card.site_customization_page_id
         redirect_to admin_site_customization_page_cards_path(page), notice: notice
       else
-        redirect_to admin_homepage_url, notice: notice
+        redirect_to admin_homepage_path, notice: notice
       end
     end