* Warn that cookies need to be activated.
* Clean up expired sessions & files from these sessions (use CRON).
* Only show options that are actually available (dependencies!).
* Expire results from celery (using TASK_RESULT_EXPIRES)
* Check celery broker timeout
* GAE: DeadlineExceededError on building the pdf does not allow to 
  save the error to the database, need another way to inform client
  that the request died
