FEATURE: Add user preference to disable smart lists (#29434)

Followup 30fdd7738e

Adds a new site setting and corresponding user preference
to disable smart lists. By default they are enabled, because
this is a better experience for most users. A small number of
users would prefer to not have this enabled.

Smart lists automatically append new items to each
list started in the composer when enter is pressed. If
enter is pressed on an empty list item, it is cleared.

This setting will be removed when the new composer is complete.

app/serializers/current_user_option_serializer.rb

@@ -4,6 +4,7 @@ class CurrentUserOptionSerializer < ApplicationSerializer
   attributes :mailing_list_mode,
              :external_links_in_new_tab,
              :enable_quoting,
+             :enable_smart_lists,
              :dynamic_favicon,
              :automatically_unpin_topics,
              :likes_notifications_disabled,

app/serializers/user_option_serializer.rb

@@ -12,6 +12,7 @@ class UserOptionSerializer < ApplicationSerializer
              :dark_scheme_id,
              :dynamic_favicon,
              :enable_quoting,
+             :enable_smart_lists,
              :enable_defer,
              :digest_after_minutes,
              :automatically_unpin_topics,