Ask confirmation to delete pages from the edit page

We were already doing that when deleting pages from the index page, and
we also ask for confirmation in almost every page in the admin section.

app/components/admin/site_customization/pages/edit_component.html.erb

@@ -7,6 +7,7 @@
     @page,
     text: t("admin.site_customization.pages.index.delete"),
     method: :delete,
+    confirm: true,
     class: "delete"
   ) %>
 <% end %>

spec/system/admin/site_customization/pages_spec.rb

@@ -93,7 +93,9 @@ describe "Admin custom pages", :admin do
     custom_page = create(:site_customization_page, title: "An example custom page")
     visit edit_admin_site_customization_page_path(custom_page)
 
-    click_button "Delete page"
+    accept_confirm "Are you sure? This action will delete \"An example custom page\" and can't be undone." do
+      click_button "Delete page"
+    end
 
     expect(page).not_to have_content "An example custom page"
     expect(page).not_to have_content "example-page"