Add dateModified property to tracks

[Noam-Bendelac]

Similar to addedBy, this will get set to the time when the track is added. This will help users be aware of the history of changes and specifically recent changes to the playlist. When a track is removed or re-added this time will be updated. This should be rendered in the frontend.

Possibly rename addedBy to modifiedBy, to match this property and so it makes sense with track removal/re-adding.