store statistics outside the dj_jobs table
The only source of webapp stats we have is the dj_jobs table.
However when a user destroys a jobs, the entry is removed from the table, and thus removed from the stats(actually not true -> the state is changed to ARCHIVED).
Also there are some privacy issues, because the current solution stores permanently the details of jobs run by every users. Ultimately the webapp statistics should not be tied to any personal data.
- decide which data we collect for generating stats
- store them in another place (not in the dj_jobs table)
In the same time it would be good to cleanup the job deletion process (remove the 'ARCHIVED' state) and address job expiration (#252).