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.

app/controllers/application_controller.rb

@@ -519,7 +519,7 @@ class ApplicationController < ActionController::Base
   end
 
   def current_homepage
-    current_user&.user_option&.homepage || SiteSetting.anonymous_homepage
+    current_user&.user_option&.homepage || HomepageHelper.resolve(request, current_user)
   end
 
   def serialize_data(obj, serializer, opts = nil)