Add `BeatmapInfo.LastUpdate` to track the time of local changes

[peppy]

Also fix DateAdded not being set to a sane value when creating a new beatmap in the editor.

Note that this date is not currently used, but I figure it will be useful for showing a user's own creations (which would default to "last modified" for sure).

• [ ] Depends on #19529 for conflict avoidance.