* It only imports users and emails so far
* It stores mapped IDs and usernames in a SQLite DB. In the future, we might want to copy those into the Discourse DB at the end of a migration.
* The importer is split into steps which can mostly be configured with a simple DSL
* Data that needs to be shared between steps can be stored in an instance of the `SharedData` class
* Steps are automatically sorted via their defined dependencies before they are executed
* Common logic for finding unique names (username, group name) is extracted into a helper class
* If possible, steps try to avoid loading already imported data (via `mapping.ids` table)
* And steps should select the `discourse_id` instead of the `original_id` of mapped IDs via SQL
