Improve Snapshot support detection and reporting
Improve snapshot directory security

The directory is accessible by owner and group only (0750)
Access to list.csv is restricted (0640)
Missing index.html triggers warning
Improve snashot file security

Snapshot file permissions set to 0640 by default
Better (longer) names for snapshot files
Listings file is now php to prevent leaking it's contents
Show notices only on the Database Export page