Move custom pages form partial to a component

This way changing it will be easier.

Note we're changing the name of the HTML class to follow our naming
conventions; the `edit_page` class wasn't used anywhere, so we don't
need to change anything else.

app/components/admin/site_customization/pages/form_component.rb

@@ -0,0 +1,9 @@
+class Admin::SiteCustomization::Pages::FormComponent < ApplicationComponent
+  include TranslatableFormHelper
+  include GlobalizeHelper
+  attr_reader :page
+
+  def initialize(page)
+    @page = page
+  end
+end