* 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
* Splits the existing script into multiple classes
* Adds command for generating IntermediateDB schema (`migrations/bin/cli schema generate`)
* Changes the syntax of the IntermediateDB schema config
* Adds validation for the schema config
* It uses YAML schema aka JSON schema to validate the config file
* It generates the SQL schema file and Ruby classes for storing data in the IntermediateDB
* Moves existing files around. All essential scripts are in `migrations/bin`, and non-essential scripts like benchmarks are in `migrations/scripts`
* Dependabot configuration for migrations-tooling (disabled for now)
* Updates test configuration for migrations-tooling
* Shorter configuration for intermediate DB for now. We will add the rest table by table.
* Adds a couple of benchmark scripts
* RSpec setup especially for migrations-tooling and the first tests
* Adds sorting/formatting to the `generate_schema` script
