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.
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