Make RecordsControllerBehavior more reusable

The primary purpose of this commit is to make RecordsControllerBehavior more reusable without requiring RecordsController or mounting HydraEditor::Engine. A secondary purpose is to improve the flexibility of the templates and partials by the addition of helpers and I18n. Incidental to making the latter modifications, the existing I18n keys have been moved from `hydra` to `hydra_editor.form` in order to avoid clashes with other existing or future Hydra components.