Conditional logging

 * Log condition for log level increase based on conditions. Once one of these
   conditions is met, the required log level is set to debug. This allows to
   debug specific requests, users or apps

 * Supported conditions (setting `log_condition` in `config.php`):
    - `shared_secret`: if a request parameter with the name `log_secret` is set to
                this value the condition is met
    - `users`:  if the current request is done by one of the specified users,
                this condition is met
    - `apps`:   if the log message is invoked by one of the specified apps,
                this condition is met
 * fix unit test and add app log condition test