Purge Jessie time-based APT snapshots

Related issues

Related to #12605 (closed)
Related to #12096 (closed)
Related to #13526 (closed)
Blocked by #10289 (closed)
Blocks #13233 (closed)
Blocked by #11738 (closed)

Original created by @intrigeri on 11912 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information