DEV: Make discourse_narrative_bot use Rails autoload (#26044)

Why this change?

Instead of manually loading files, we should just structure the plugin
so that it relies on Rails autoload strategy and avoid all the manual
`require_relative`s.

What does this change do?

1. Structure the plugin to use Rails autoloading convention
2. Remove onceff jobs that were added 5-6 years ago. There is no need to
   carry these jobs anymore after such a long time.
3. Move setting of `SiteSetting.discourse_narrative_bot_enabled` to
   `false` in the test environment from core into the plugin.

plugins/discourse-narrative-bot/lib/discourse_narrative_bot/engine.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+module DiscourseNarrativeBot
+  class Engine < ::Rails::Engine
+    engine_name PLUGIN_NAME
+    isolate_namespace DiscourseNarrativeBot
+    config.autoload_paths << File.join(config.root, "lib")
+  end
+end