This commit main goal was to comply with Zeitwerk and properly rely on autoloading. To achieve this, most resources have been namespaced under the `Chat` module.
- Given all models are now namespaced with `Chat::` and would change the stored types in DB when using polymorphism or STI (single table inheritance), this commit uses various Rails methods to ensure proper class is loaded and the stored name in DB is unchanged, eg: `Chat::Message` model will be stored as `"ChatMessage"`, and `"ChatMessage"` will correctly load `Chat::Message` model.
- Jobs are now using constants only, eg: `Jobs::Chat::Foo` and should only be enqueued this way
Notes:
- This commit also used this opportunity to limit the number of registered css files in plugin.rb
- `discourse_dev` support has been removed within this commit and will be reintroduced later
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