When an extension or a version is deleted, create a `LogEntry` for each record linked to that extension or version.
Each `LogEntry` has action flag `DELETION` and contains a blob with the last known field values of the records.
These `LogEntry` can be viewed by admins in `/admin/admin/logentry/`, same as any other `LogEntry` created by Django admin, so this should be sufficient for archival purposes.
Part of #82
Reviewed-on: #84