Modification performed for service account

JIRA INFRA-2909 #action In Progress

Retrieving all items from the database takes some time.
The task is thus handled by the RQ worker.
We use ajax to periodically monitor the status of the job
and redirect to the file when ready.
A progress bar is displayed to let the user see the progress.

The mechanism put in place could be re-used for other use cases.

JIRA INFRA-545 #action In Progress