Fix errors during backup task. Fix #3785

By default there is no public/uploads directory when no attachments
are uploaded. Prompt users to create the uploads directory during
install otherwise the backup task will fail.

Place mysqldump args in single quotes to avoid error if password
contains special characters.

Signed-off-by: Axilleas Pipinellis <axilleas@archlinux.gr>
Merge pull request #3871 from axilleas/backup-fix

Fix errors during backup task. Fix #3785