Variant snapshots should be cleaned way more often

[MartinBelthle]

Problem Statement

Currently, the function clear_snapshot which removes the snapshot directory (it contains the real files of a variant study) is only called in the auto_archive_service.py module.

Therefore, they are only cleaned once a while according to the field auto_archive_threshold_days : 60 days by default and 120 days for recette env. !!

Proposed Solution

Create a new field in the application.yaml which concerns only variants with a default value of something like 1 day.