Cleanup account with an expired validation token (Relates #37)

Cleanup not activated user account after a while. The cleanup is ensure by a new cronjob run by the worker.

It runs once every hour.

"A while" is customizable by the config key token_expiration_days and default to 7 days

This MR adds unit tests for this cleanup tasks including a new Class TestCronjob that can be used to tests other cronjobs.