Admin: print a backup reminder before database migration

[wajda]

Before starting a migration procedure, print a bold message suggesting creating a database backup.
Something like the following:

admin-cli db-upgrade ....

**************************************************************
**                      WARNING !                           **
** This operation is not reversible! Please make sure       **
** you created a full database backup before continue       **
**                                                          **
** Instruction how to make backups can be found here:       **
** https://www.arangodb.com/docs/stable/backup-restore.html **
**************************************************************

I confirm that I created a database backup: [y/N]

The message and the prompt should only appear when CLI is executed in the interactive mode.