Mark slots as old when users aren't deleted

[itsrachelfish]

Currently if the schedule is changed after a user has already signed up for a shift, the slot they signed up for won't be deleted. This is useful to ensure that users who signed up aren't randomly deleted from their shifts, but it creates a situation where there can be multiple duplicated shifts that don't appear in the schedule on desktop but can still be viewed on mobile. This also impacts reports as the duplicated slots also appear when reports are generated. See issue #247 for example

A solution to this would be to mark all old slots with a database flag when the schedule is updated. This would make it trivial to differentiate between which slots are the most recent and which ones are old with users who previously signed up.