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.

lib/homepage_helper.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+class HomepageHelper
+  def self.resolve(request = nil, current_user = nil)
+    return "custom" if ThemeModifierHelper.new(request: request).custom_homepage
+
+    current_user ? SiteSetting.homepage : SiteSetting.anonymous_homepage
+  end
+end