Before, I used request.session to store some information about the current jobs to be printed.
This was because in the very beginning, I had an issue with disappearing files in my model.
I was able to fix this issue in a previous commit, but I still needed the jids.
However, because of this storage system, if someone tried to print a file, then oppened a second tab,
and tried to print anoter file, he/she would pay twice for the second file ....
I decided to add a hidden field to store the job id and deal with it, and it is readonly to avoid
changing it.
