MDL-12403 backup: Ability to exclude question bank

Now users can choose to exclude the question bank from the backup,
this will in turn prevent any modules which use the question bank
from being included in the backup.
MDL-12403 backup: configure question bank in automated backups
MDL-12403 backup: ability configure default for question bank excludes
MDL-12403 backup: force config of questionbank settings

To prevent upgrades causing incomplete backups.
MDL-12403 backup: prevent unset config from being applied