Backend user management

* Found some labels still in German and translated to English
* Extended the Validator class to support custom validation rules and rules from the ValidationRules class
* Changed/Fixed backend design (form elements, buttons, form errors, default table layout)
* Form::input() now renders superfluous options as html attributes
+ Added ValidationRules class
+ Added user management to the backend
+ Added methods count() and delete() to DB class
+ Added methods create(), update() and delete() to User class
+ Added method Router::redirect()
+ Added method Form::value()
+ Added default backend user (admin)
+ Added links that need confirmation (like deleting)