snapshots: fixes cleanup of old snapshots

Snapshot cleanup did not work due to time.Now syntax error. Added test
for it as well to catch any future errors.

Added error and debug logging so that it is possible to see any errors in the future.

Removed an unused configuration value and deprecated the remove expired snapshots
setting.

pkg/models/dashboard_snapshot.go

@@ -64,6 +64,7 @@ type DeleteDashboardSnapshotCommand struct {
 }
 
 type DeleteExpiredSnapshotsCommand struct {
+	DeletedRows int64
 }
 
 type GetDashboardSnapshotQuery struct {