Bug 1084014 - [Calendar] update Month view to use DayObserver r=gaye

### Changelog

 - merge important logic from `TimeParent` into `Month` to keep a shallow inheritance tree and a simpler structure.
 - use `dayObserver` and create a `MonthDay` view that is only responsible for the busytime dots.
 - this patch will degrade the performance since it does way more calls to `timeController.findAssociated` than the old logic, but this is addressed by the new `dayObserver` introduced by #26511
this was addressed by #26511 so we should not care about it here.. both patches should land during same day/week, so no big deal.