MigrationConfigValidator.php, ValidationResult added and services.yml updated

Closes #3579149. Add migration config validator and logger for wizard UI and drush users alike. Introduce new logging service + updated validator. New: Constants.php, LogEntry.php, MigrateLogger.php, ThresholdLogger.php, wordpress_migrate.settings.yml for config and log control.Modified WordPressMigrationGenerator.php, WordpressMigrateCommands.php, wordpress_migrate.services.yml, ValidationResult.php renamed, MigrateConfigValidator.php, LogTerm.php (docs: processing-plugins.md, Readme.md updated, error-logging.md).