DEV: Adds a basic importer for the IntermediateDB

* 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

migrations/lib/cli/import_command.rb

@@ -9,10 +9,9 @@ module Migrations::CLI
     end
 
     def execute
-      ::Migrations.load_rails_environment
+      ::Migrations.load_rails_environment(quiet: true)
 
-      puts "Importing into Discourse #{Discourse::VERSION::STRING}"
-      puts "Extralite SQLite version: #{Extralite.sqlite3_version}"
+      ::Migrations::Importer.execute
     end
   end
 end