DEV: allow themes to render their own custom homepage (#26291)

This PR adds a theme modifier and route so that custom themes can opt to show their own homepage. See PR description for example usage.

db/migrate/20240326200232_add_custom_homepage_theme_modifiers.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class AddCustomHomepageThemeModifiers < ActiveRecord::Migration[7.0]
+  def change
+    add_column :theme_modifier_sets, :custom_homepage, :boolean, null: true
+  end
+end