Execute a full backup of GlobaLeaks before upgrading it

[fpietrosanti]

Current behavior
Actually we suggest the execution of a manual backup before the upgrade of globaleaks:
https://globaleaks.readthedocs.io/en/latest/BackupGuide.html

Expected behavior
This ticket is to execute a full backup of GlobaLeaks before upgrading it, after the installation of a new package, in a full automatic way.

This ticket is to delete the backup only after X days following the upgrade (es: 7 days), in order to gives the sysadmin a way to detect possible malfunctions.

The configuration of the amount of time the automatic pre-upgrade backup has to be kept, has to be configurable.

[evilaliv3]

In commit 3b497f6 i've implemented a first step for this.

• An automatic backup is performed during the debian pre-inst if an existing setup of globaleaks is detected
• The backup is stored in /var/globalealsks/backups for 15 days and then removed by globaleaks in the cleaning scheduler
• The backup file has the format like globaleaks-2.72.27-1518305224.tar.gz (globaleaks-$version-$timestamp.tar.gz