If the user hasn't set the version_at column already, it should be assigned to now.

This fixes the bug that a new version would keep the same version_at value as the
previous version.

Signed-off-by: Chris Riddoch <criddoch@globe.gov>
Add validators in to require version_at_column and comments.

Signed-off-by: Chris Riddoch <criddoch@globe.gov>
Including validators from master

Signed-off-by: Chris Riddoch <criddoch@globe.gov>