FEATURE: Allow the user to select a custom home page (#5268)

* Add user_home configuration option

* Use the new user_home preference to actually show the right home page

* Fix trailing whitespace

* Update user_option_serializer.rb

* Fix JavaScript default homepage tests

* Use an object instead of a giant switch

* Remove trailing whitespace

* Make the default `user_home` set to `null` instead of `0`

* Rename user_home to homepage_id

app/controllers/application_controller.rb

@@ -308,7 +308,7 @@ class ApplicationController < ActionController::Base
   end
 
   def current_homepage
-    current_user ? SiteSetting.homepage : SiteSetting.anonymous_homepage
+    current_user&.user_option&.homepage || SiteSetting.anonymous_homepage
   end
 
   def serialize_data(obj, serializer, opts = nil)