Implementation of backup functionality based on rsync

[evilaliv3]

This ticket is related to the analysis and integration of the backup functionality provided in #4314

The feature make it possible for the web administrator to configure a backup pathwhere to perform a backup copy of the directory /var/globaleaks

[evilaliv3]

Status of the current implementation:

• From the demo of the functionality the functionality seems fully functional.

Actions necessary to possibly integrate same functionality:

• Very minor corrections seems to be needed making it possible to configure the backup settings before enabling it
• Very minor corrections seems to be needed to make to configure an rsync endpoint instead of local path making it possible to satisfy Automatic Remote Backups  #528 without need for a sysadmin to map a local system path on a remote disk at system level
• Implement some client or backend unit tests to guarantee appropriate code coverage

[evilaliv3]

I've proceeded with the full revision of the proposed changes to database included in this implementation.

The revisions consider to be just changes in naming necessary for globaleaks for international naming conventions and code consistency that are:

• Performed code review of the variables name choice to keep codebase uniform

We have avoided other structural changes understanding this could have major requirements in the refactoring of the code proposing the minimum changes necessary and proposing only the changes already previously indicated.