backups: Error if export location does not have valid label

NoReverseMatch at /sys/backups/

Reverse for 'restore' with arguments '('/dev/sdb', 'FreedomBox_backup_2018-09-242158.tar.gz')' not found. 1 pattern(s) tried: ['sys/backups/restore/(?P<label>[^/]+)/(?P<name>[^/]+)/$']

In this case, we should probably quote the device name to make it a valid part of URL.